COMMENTS ON THE WATER VASCULAR SYSTEM, FOOD GROOVES, AND ANCESTRY OF THE CLYPEASTEROID ECHINOIDS

Transcription

1 BULLETN OF MARNE SCENCE, 27(3): , 1977 COMMENTS ON THE WATER VASCULAR SYSTEM, FOOD GROOVES, AND ANCESTRY OF THE CLYPEASTEROD ECHNODS Thomas F. Phelan ABSTRACT Schemes for distribution of wat~r vessels in adoral, ambital, and petaloid regions of the ambulacra are described. n the adoral and ambital region three basic patterns are described. All three function as connections between the accessory tube foot ampullae and the radial water vessels. Lobe-like expansions or side spurs of the radial water vessels connect to the accessory tube foot ampullae in genera of the suborder Laganina. Simple unbranching lateral water vessels connect to the accessory tube foot ampullae in genera of the suborder Clypeasterina. Complex branching lateral water vessels were observed in genera of the suborder Scutellina. Accessory tube feet help collect food and are closely associated with the food grooves. Four schemes of lateral water vessel and accessory tube foot distribution within petaloid areas are described, two observed in petals composed of primary plates only, and two observed in petals with primary and demiplates. The respiratory tube foot/ampulla system of Del/draster excentricus is comparcd with the suckered tube foot/ampulla system of the regular urchin Strongylocentrotus purpuratus. The ampullae of both systems are flattened, and very thin walled. Fluid constantly circulates between tube foot and ampulla. Un flattened bulbous ampullae were observed on the accessory tube feet and buccal tube feet of clypeasteroids and the buccal tube feet of the regular urchin S. purpura/us. These bulbous ampullae connect to the respective tube feet through single pores. The greatly expanded ambulacral columns and adoral interruption of interambulacral columns are considered closely related to the development of the accessory tube foot system in clypeasteroids. These expanded ambulacra and the adjacent regions of the interambulacra which support accessory tube feet are considered phyllodes and homologous to the more recognizable but less expansive phyllodes of the cassiduloids. Some characters are common to the cassiduloids and clypeasteroids strongly suggesting a cassiduloid ancestry for the clypeasteroids. Among these are a monobasal apical system, respiratory pores between petal plates, the shape of lantern pyramids, buccal tube feet on basicoronal plates, expansion of the ambulacra forming phyllodes, and single pore supported tube feet adoral to the petals. Most previously published works that mention accessory tube feet are limited to accessory pore distribution, tube foot activity, and histology. n this paper intend to describe the several schemes of water vessel distribution and the functional relationship of accessory tube feet to food grooves where present. Comments on the increase in number of accessory tube feet and lateral water vessels relative to plate growth are limited as the processes by which this is accomplished is not yet fully understood. This study is continuing on the plates of Dendraster excentricus. Less extensive comments are given on the respiratory tube feet/ampullae and the ampullae of buccal tube feet. The relationship of accessory tube feet to expanded ambulacral columns and the recognition of these and portions of some interambulacra as phyllodes lead to the comments on the ancestry of clypeasteroids. Many of my ideas on the relationship of clypeasteroids to cassiduloids had as their source my close association with Porter M. Kier. 400

2 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 401 MATERALS The echinoids listed below were studied for this work. Throughout the text generally only the generic name is used because this paper is primarily concerned with principles rather than systematic descriptions. Echinocyamus sp. USNM ] 406 (3 specimens). Jacksonaster fudsiyama (Doederlein) USNM 34]96 (l specimen), Heliophora orbiculus (Linnaeus) USNM (1 specimen), Clypeaster subdepressus (Gray) USNM E578 (2 specimens). C. rosaceus (Linnaeus) (l specimen), Pel/aster zelandiae (Gray) USNM E0140 (1 specimen), Encope michelini Agassiz (5 specimens), E. aberrans Martens (2 specimens). Mellita quinquiesperforata (Leske) (2 specimens), Dendraster excentricus (Eschscholtz) (3 specimens from San Francisco Bay, 15 specimens from off Coos Bay, Oregon). Echinarachnius parma (Lamarck) (3 specimens). Strongylocentrotus purpuratus Stimpson (6 specimens). The specimen of the second species listed above (Jackson aster fudsiyama) is catalogued as Laganum fudsiyama Doederlein but due to the shape and position of the periproct and large size of the first post-basicoronal plates in interambulacrum 5, feel more comfortable referring it to the genus Jacksonaster. This is of little significance though as the water vessel scheme for serving the accessory tube feet is most likely similar in all the laganids. Definition of Terms The Lovenian system based on columns of plates is used in this paper to define the boundaries of the ambulacra and interambulacra. Clypeasteroids commonly have tube feet in the interambulacra. To avoid confusion the distribution of tube feet is not used as the basis for establishing the boundaries of the ambulacra. Throughout this paper, plate sutures are referred to by name and four terms of direction or position are commonly used. These are defined as: interradial Slltllre, a midline (meridional) suture between two columns of interambulacral plates; adradial suture, a suture on the boundary between an ambulacrum and an interambulacrum; perradial suture, a midline (meridional) suture between two columns of ambulacral plates; adapical suture, the transverse (horizontal) suture on the adapical edge of a plate in a column; adoral suture, the transverse (horizontal) suture on the adoral edge of a plate in a column; adapical/adoral sutures, the transverse (horizontal) sutures of a column of plates. The adoral suture of one plate is the adapical suture of the adjacent older plate of that column; adapical, a direction or position toward the apical system following the course of the column of plates; adoral, a direction or position toward the mouth following the course of the column of plates; adradial, a direction or position toward the perradial suture; abradial, a direction or position away from the perradial suture. Note: The adradial sutures are on the adradial edges of the interambulacra, but are on the abradial edges of the ambulacra. THE MADREPORTE, TUBE FEET, AND AMPULLAE Madreporite The apical system of clypeasteroids is monobasal, there is a single large genital plate. This plate also serves as the madreporite. The development of this plate is quite varied among the clypeasteroids. The complexity of the madreporite parallels the complexity or sophistication of other test characters such as internal supports, food grooves, and pore pairs (e.g. simple round non-conjugate or elongate conjugate and partitioned). Ontogenetic changes occur in the madreporite concurrently with development of other test characters. n many genera with simple test characters, fossil and Recent, a single pore in the madreporite is common (e.g. Echinocyamus). More sophisticated forms have a few more pores. Some genera have pores in little grooves (e.g. Laganum). Clypeasteroids with highly

3 402 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, 1977 sophisticated tests commonly have a large star shaped madreporite with hundreds of pores (e.g. Encope). g Primary Tube Feet The first tube feet to develop on all echinoids are the five primary tube feet present at metamorphosis Hyman, 1955: 511, figs. 219a,b). Respiratory tube feet have been called primary tube feet by some authors, but to avoid confusion with the five tube feet present at metamorphosis they are called respiratory tube feet in this study. Respiratory Tube Feet The petaloid area of each ambulacrum is the most conspicuous external feature associated with tube feet in c1ypeasteroids. The pore pairs in the petals are situated between plates and not within plates as is common with other echinoids. The pore pairs at the distal ends of the petals of some genera are within plates (e.g. Jacksonaster). The petal pore pairs serve respiratory tube feet. The outer (abradial) pore is subdivided or partitioned in many genera (Fig. 1A). Respiratory tube feet have no suckers, are flattened in an adapicaljadoral direction but transversely elongated in spanning the respiratory pore pairs. The ampullae which serve respiratory tube feet are similarly flattened. They taper slightly as they extend into the perivisceral coelom. The blind ends of the ampullae were attached to the gonads in "ripe" specimens of Clypeaster, Mellita, and Dendraster. They are probably also attached to the gonads in many other genera. Nichols (1959:551 and 1961:177) reported a complete lack of ampullae on the respiratory tube feet of Echinocyamus pusi/- us (Muller). However, ampullae were found on the respiratory tube feet of all clypeasteroids with preserved tissue that were examined in this study including specimens of Echinocyamus sp. (USNM 14046) from Unalaska, Alaska. There is a continuous circulation of coelomic fluid through the ampullae and tube rtf Fig.. (A) Dendr(/ster excentriclls, respiratory tube foot (rtf) has division for circulating fluid (arrows), and a globular tip (g) on the finger-like protuberance above the outer pore, flattened ampulla (a) with out pouchings (op). The inner edgc of the ampulla is at the inner pore. The ampullac for the accessory tube feet (atf) are in a cavity in the stereom near the radial water vesscl (r). (B) Strongylocentrotus purpuratus, the tube foot (tf) has a partition near its base, the ampulla (a) is flattened, interconnections between ampullae sides form passageways for the circulating fluid (arrows). The inner edge of the ampulla is between the inner pore and the radial watcr vessel (r). Both figures are diagrammatic. feet. The fluid enters the tube foot through the outer (abradial) pore and returns to the ampulla through the inner (adradial) pore. The flow of coelomic fluid can be determined by observing the movement of coe- 10mcytes past a given point.

4 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 403 observed this circulating fluid in Dendraster excentricus (Eschscholtz) and compared it with that of the regular echinoid Strongylocentrotus purpuratus Stimpson. The current in both species moved in the same direction and seemed to flow at the same rate. n D. excentricus a respiratory ampulla has two broad flattened sides. These sides have out pouchings which increase the surface area (Fig. la). S. purpuratus also has ampullae with flattened sides. There are several rows of connections between the two sides of each ampulla. The circulating fluid was observed flowing between these rows (Fig. ] B). The inner edge of an ampulla is between the radial water vessel and the inner pore. n D. excentricus and other clypeasteroids examined the inner edge of each ampulla was at the inner pore. A comparison between the tube foot and ampulla of D. excentricus and S. purpuratus is shown diagrammatically in Fig. 1. observed very little action of the ampullae in S. purpuratus even though the tube feet were active. The most noticeable activity was the constant current circulating between the ampullae and tube feet. did not observe any fluid entering the tube foot through the inner pore. The current in this pore was always from the tube foot to the ampulla. believe it is to facilitate this circulating current that echinoids have two pores for some tube feet. Apparently this circulating current was present even in early Paleozoic echinoids since they have a pair of pores for each tube foot. Externally there is a finger-like protuberance of the respiratory tube foot above the outer (abradial) pore on D. excentricus. The protuberance terminates with a small hollow globular tip (Fig. la). When touched the tip of a protuberance with a barbed broach the tube foot immediately retracted and the surrounding broad tipped spines closed protectively over the entire respiratory tube foot. The response to touch was not conducted on other areas of the tube foot. Similar protuberances were not seen on the respiratory tube feet of Encope michelini. have not observed the respiratory tube feet of live specimens of other c1ypeasteroids under the microscope. MacBride (1909: , fig. 242) described and figured the respiratory tube feet of Echinarachnius parma and reported suckers at their tips. The globular tip on a respiratory tube foot of D. excentricus is not a sucker. Buccal Tube Feet Some c1ypeasteroids have buccal tube feet. An un flattened bulbous ampulla serves each tube foot, through a single pore in the peristomial edge of each basicoronal ambulacral plate. There are 10 plates, each with a buccal tube foot, on the peristomial membrane of many regular echinoids. Nichols (] 961 : 161) termed these tube feet "peristomials" and reported they had no ampullae. The buccal tube feet/ampullae (peristomials) of the regular urchin Strongylocentrotus purpuratus were studied for comparison with the buccal tube feet/ampullae of the dypeasteroids. found the buccal tube feet of S. purpuratus to have fusiform shaped ampullae. Accessory Tube Feet Thousands of very small single pores which serve suckered accessory tube feet are distributed throughout the ambulacra of dypeasteroids. These pores are difficult to see with the unaided eye. n some genera the accessory tube feet are distributed from the adapical end of the petals to the peristomial edge of the test. n other genera the accessory tube feet are distributed widely in the interambulacra as well. The histology of the accessory tube feet and ampullae of Echinocyamus pusillus was described by Nichols (1959: , figs. 2-5). The external appearance of the accessory tube feet of four species is given below. Clypeaster rosaceus (Fig. 2) has relatively

5 404 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, #, ':~,:\.~~':/::;.;:':'< ~~.. -. :~.~ "":1.0' i:.,.... _ "0." A '0 :'~.\':~;.'~~':",.~ Figure 2. Terminal end of an accessory tube foot of Clypeasler rosacel/s. large accessory tube feet. The diameter of the terminal knob including the prominent sheath is some 144 j.tm. A diaphragm some 36 ftm in diameter is at the tip. On fixed specimens it protrudes some 6 ftm. The accessory tube feet of C. rosaceus are considerably larger than those of Echinocyamus described by Nichols (1959: , figs. 2,3) but many of the features are quite similar. On Dendraster excentricus, Encope michelini, and Mellita quinquiesperjorata there are longer accessory tube feet (retracted state) in the areas immediately bordering the food grooves than elsewhere except for the large diameter accessory tube feet in the food grooves near the peristome. The long tube feet bordering the food grooves of D. excentricus are about twice the diameter of those commonly found in the food grooves, remote from food grooves on the adoral surface, and in the petals. The terminal knobs ~ 15J.l ----i B Figure 3. Terminal ends of accessory tube feet of Dendraster excelltricl/s showing the changes in shape of tip. (A) Terminal knob closed concealing most of the conical tip. (B) Terminal knob parc tially withdrawn from conical tip. (C) Terminal knob fully withdrawn from conical tip. n this position the conical tip is very prominent.

6 ::: :: PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 405 '., "':::!:}':;:~ft...,.' ~.",,;c.~ b, ~\''':''',,\/ i). _~i. 0 -;' ' 0 ~.~ ~ ~.i-':\ ':.'.. ' '..... :." :..;~.. '0,' 0 i Figure 4. Terminal end of an accessory tube foot of Encope michelini. of the larger tube feet near the peristome and bordering the food grooves of D. excentricus are some 120 p'm in diameter. The common smaller tube feet have terminal knobs some 60 to 72 p'm in diameter. Live specimens of D. excentricus were readily available for study and considerable time was spent observing tube foot activity. The accessory tube feet extend slowly with a waving motion then retract very rapidly. The terminal knobs of the accessory tube feet appeared capable of altering their shape. Sometimes a very prominent conical tip extended beyond the terminal knob and again it would be difficult to detect. Serial sections 6 p'm thick were made on numerous accessory tube feet to determine the nature of these changes. Unsectioned accessory tube feet were also photographed at various angles to help in this determination. t was found that the conical tip was always present but was only prominent when the dense terminal knob was withdrawn from around it (Fig. 3A-C). A diaphragm or depression is not apparent on the tip of the terminal knob of the accessory tube feet of Encope michelini or Mellita quinquiesperjorata. The capability of these tube feet to lift relatively large shell fragments could indicate that some form of sucker tip is present. The research cine films produced by Kier in 1972 show closeup views of this activity. The shape of the terminal knob of the accessory tube feet of Encope and Mellita is very similar so only that of E. michelini is illustrated (Fig. 4). The diameter and height of the terminal knob of accessory tube feet from several selected positions on E. michelini are given below. a. Submarginal not in a food groove: diameter 60 p'm, height 48 p.m. b. Main branch of a food groove: diameter 72 p'm, height 60 p.m. c. Near peristome in perradial portion of food groove: diameter 120 p'm, height 72 p.m. The diameter and height of the terminal knob of the tube feet from several positions on M. quinquiesperjorata were measured. a. Main branch of a food groove: diameter 84 p'm, height 60,.,.m. b. Remote from a food groove: diameter 84 p'm, height 60 p.m. c. Near peristome in perradial portion of a food groove: diameter 132 p'm, height 84 p.m. The ampullae of the accessory tube feet are variable in shape and size. Those of live specimens of D. excentricus were observed (Fig. 5). They are almost cylindrical except for a constriction where attachment is to the lateral water vessel. The blind ends of the ampullae were spherical and all had a globular cluster of coelomocytes in the spherical tip. The shape of the ampullae gradually changed somewhat after fixation in 70% ethyl alcohol. The spherical shape of the blind end was lost. The globular mass of coelomocytes in each ampulla broke up. They fragmented and the parts drifted about

7 406 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, ] JJ Figure 6. Simple perradial food grooves (fg). Grooves of this type are found in the laganids, Clypeaster, and Fel/aster but those of Pcl/aster extend over the ambitus to the apical system. are ingested for the attached organic material. n other genera such as Dendraster, organic material and little sand is ingested. Figure 5. Accessory tube foot ampulla of Delldraster excclltricus, the finely stippled area in the spherical blind end and cylindrical portion represent masses of coelomocytes in the fluid. Lateral water vessel added to show position only. t was not in the photograph. in the ampullae. While alive the spherical blind ends had a dark appearance due to the color of the cluster of coelomocytes. RELATONSHP BETWEEN WATER VESSELS, ACCESSORY TUBE FEET, AND FOOD GROOVES OF THE AMBTUS AND ADORAL SURFACES Accessory tube feet help collect food or food bearing material (sand grains) and pass it toward the mouth. n genera such as Clypeaster, Mellita, and Encope, sand grains Food Grooves A food groove is a furrow on the external surface of the test of an echinoid leading toward the peristome. Particles bearing organic material and free organic material are passed to the mouth along the food grooves. n some genera accessory tube feet are abundant within and around the food grooves. n other genera few or no accessory tube feet occur within the food grooves but are abundant in surrounding areas. Food grooves with accessory tube feet commonly have an abundance of glassy tubercles which bear no appendages. The glassy tubercles and accessory tube feet extend in a series beyond the point where the food groove ceases to be a furrow. There are two basic kinds of food grooves with many intermediate types. The perradial food groove (Fig. 6) is the simplest form and the polyfurcating food groove the most complex (Fig. 13). The basic perradial food groove is a simple unbranching furrow lying along the

8 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 407 perradial suture between two columns of ambulacral plates. Each radial water vessel lies along a perradial suture on the interior of the test. The water vessel is separated from the suture by other radial systems such as the radial nerve. There are obstacles to overcome in supplying accessory tube feet directly in the perradial food groove due to the position of the radial systems. This problem has been solved in different ways which will be discussed later in this section. Adorally the polyfurcating food groove has a very short section lying on the perradial suture ncar the peristome. Beyond this short section the food groove divides into two main branches. Each branch extends along the growth centers of a column of ambulacral plates. There are commonly many side tributaries to each main branch and additional fllrcations near the ambitus. The main branches and tributaries tend to cross plate sutures at nearly right angles. The major portion of a polyfurcating food groove is remote from the internal position of the radial systems. This provides unobstructed access for connections between the internal lateral water vessels and the accessory tube feet in the food grooves. Service to the accessory tube feet in the short perradial portion near the mouth must overcome the same difficulties noted for the simple perradial food groove. This willbe discussed later in this section. PoLyfurcating food grooves provide a greater network of food gathering channels to the mouth than perradial food grooves. The fibulariids lack food grooves and have the simplest scheme in the clypeasteroids. Radial Water Vessels with Side Lobes or Spurs The fibulariid Echinocyamus has relatively broad radial water vessels for its small size. Accessory tube foot ampullae are in a band on each side of the radial water vessel on the basicoronal ambulacral plates. There is a lobe-like expansion of the radial water vessel onto each younger ambulacral plate (Fig. 7). A band of accessory tube foot ampullae lmm Figure 7. Radial water vessel (r) of Echillocya- 1US showing the lobes () tbat serve the accessory tube foot ampullae. Position of ampullae indicated by large dots, peristome (p), ambitus of test (t). border each lobe except on its adoral side (side closest to the peristome). The pore pattern of the exterior adoral surface reflects the position of accessory tube foot ampullae. Nichols (1959 :fig. 4a) shows the connection of an accessory tube foot/ampulla to the radial water vessel. The internal surface of the adoral plates of the test is sculptured in such a way that the position of the radial water vessel and side lobes is evident even on denuded specimens. This is well illustrated in Durham (1966b:469, fig c). The exterior accessory pore pattern reflecting the radial water vessel shape for E. pusillus is shown in Durham (1966b:469, fig b). The pore pattern of the basicoronal and first few post-basicoronal plates of Durham (1966b:455, fig ) and Nichols (1959:541, fig. b) is inaccurate. The accessory pores in Echinocyamus are in bands parallel to the perradial suture on basicoronal plates. At the ambitus the bands of pores are at right angles to the perradial suture. The bands of pores between these two positions are transitional in position and

9 408 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, 1977 ;.'.\ :: \ ::. \......'..'.'....,~..., "' ,----- " " r.. ".-.,~.. "..'.,,". ':.". ~' 11 J. ~. " #, ~:.. ~':'~.. f', ". ~'.: '," ::: t..\ 2mnl Figure 8. View of the internal surface of a portion of a test of Jacksonaster fudsiyama showing radial water vessel (r) with one side spur (sp) on the adoral edge of each post-basicoronal ambulacral plate. There is a dense band of accessory tube foot ampullae (stippled area) along the sides of the radial water vessel and side spurs. Dashed lines are plate sutures. are in more or less arcs. The accessory pores between the ambitus and the petaloid areas are on the adapical plate sutures (between ambulacral plates of a column). Middle Eocene fibulariids apparently also had these lobes on the radial water vessels to serve accessory tube feet. Note the accessory pore patterns of Echinocyamus hi- sexus Kier (1968: 19, fig. 21) and Leniechinus herricki Kier (1968: p. 1, fig. 4). The aganids and fibulariids are members of the suborder Laganina. The genus Jacksonaster (a laganid) has simple unbranching perradial food grooves that extend from the peristome partway to the ambitus (Fig. 6). The radial water vessels and dense bands of accessory tube foot ampullae of Jacksonaster (Fig. 8) are very similar to those of Echinocyamus. The radial water vessels have long blind ending side spurs. One spur lies along the adoral edge of each post-basicoronal plate. There are no side spurs on the basicoronal plates, only a dense band of accessory tube foot ampullae on each side of the radial water vessel as in Echinocyamus. On each post-basicoronal ambulacral plate the accessory ampullae are in a continuous band along the side of a radial water vessel and one side and tip of a spur. No ampullae are connected to the side of the spur that lies near the adoral plate suture (Fig. 8). There is an abundance of accessory tube feet in the perradial food grooves of Jacksonaster. The test is thin at the perradial sutures because internally the radial systems lie in a groove on each perradial suture. The accessory tube foot ampullae close to each radial water vessel connect to the tube feet in a perradial food groove by passing obliquely through the test. The side spurs are not directly opposite each other (Fig. 8) due to the offset nature of the ambulacral plates on each side of the perradial sutures. This is because the basicoronal plates are of unequal length. The offset position of the side spurs allows for an uninterrupted series of accessory tube feet in the food grooves. do not consider the lobes of Echinocyamus or the side spurs of Jacksonaster true lateral water vessels. They are broad and flattened like the radial water vessels. Simple Lateral Water Vessels Clypeaster and Fellaster (suborder elypeasterina) have simple systems of true lat-

10 PHELAN: WATER VASCULAR SYSTEM OF ECHfNOJDS 409 '.. ",. '",".,'... mm ~~w\' -----l mm Figure 9. View of internal surface of a portion of some adoral ambulacral plates of Clypeaster subdepressus showing radial water vessel (1'), lateral water vessels (lwv) and position of accessory pores/ampullae (a). Some lateral water vessels lie across plate sutures (dashed lines). eral water vessel distribution. Many lateral water vessels connect the accessory tube feet of each plate to a radial water vessel. Clypeaster has the simpler system of the two. The lateral water vessels lie on the internal surface of the plates commonly parallel to the adapical and adoral plate sutures. A few lateral water vessels lie across adoral or adapical plate sutures and serve accessory tube feet/ampullae of two ambulacral plates. Some lateral vessels lie across adradial sutures and serve accessory tube feet/ampullae of an ambulacral and an interambulacral plate (Fig. 9). have not observed branching of these simple vessels. n some species (e.g. C. rosaceus) the stereom partially or completely closes over many lateral water vessels. Many accessory tube foot ampullae are connected to each lateral water vessel. All ampullae observed were connected to the adapical side of the vessels. No accessory tube foot ampullae were observed directly connected to a radial water vessel. The number of lateral water vessels and accessory tube feet on a plate increases as a plate expands with growth, but the processes by which this is accomplished is not yet fully understood. 'i\ fg Figure 10. External view of an adoral area of test of Clypeaster sllbdepresslls showing a portion of the food groove (fg) and perradial suture (ps). Notch on each accessory pore points in an adoral/ ad radial direction where lateral water vessels are at right angles to a perradial suture. Note the absence of accessory pores in the perradial food groove. Clypeaster has simple perradial food grooves that almost reach the ambitus. Accessory tube feet are numerous and extensive on either side of the food grooves (Fig. 10). Only near the ambitus were accessory tube feet observed within the food grooves. nternally the radial systems lie in a groove along each perradial suture. There are many lateral water vessels on each ambulacral plate on the oral surface of Fellaster but their pattern is slightly more complex than in Clypeaster. On the internal surface of the basicoronal plates the lateral water vessels lie at an angle toward the peristome. n the area of the post-basicoronal plates, lateral water vessels are inclined slightly toward the ambitus. f these vessels branch it is not commonly. have not observed it. The number, position, or branching of bands of accessory tube feet/pores on the exterior surface does not reflect the number, position, or branching of the lateral water vessels. The exterior surface pattern of accessory pores is accomplished by realignment of the pores as they pass to the exterior surface of the test (Fig. 11). The center of the exterior surface of a third or

11 :::::: BULLETN OF MARNE SCENCE, VOL. 27, NO.3, _ ~ _.." ---~=~:=-::~:-::~~~!~;_::~: ~ _..;o c; o.o.o;; ~ ~oti~ ~ J p /. /... -' /~ ~~..,ri".... M" mm i-=---l Figure 11. Fellaster zelandiae, rows of accessory pores (stippled bands) on a portion of an adoral ambulacral plate. Circles representing tubercles are drawn around one point of branching (omitted elsewhere). Number, position, and branching of pore bands do not represent number, position, and branching of lateral water vessels. Food groove (not shown) is to the right. Pcristome is in direction of arrow (p). fourth postbasicoronal plate commonly has at least one band of accessory pores with several branches but on the interior, the lateral vessels are unbranching and parallel to each other. The lateral water vessels do not extend into the immediate regions of the adradial sutures. The distal ends of the vessels are in microcanals. have been unable to determine if the ampullae are connected to the water vessels in a set pattern as in Clypeaster. Lateral water vessels and accessory tube feet become more numerous as plates expand with growth but it is not fully understood how this is accomplished. The unbranching food grooves of Fellaster lie along the perradial sutures from near the apical system to the peristome. No accessory tube feet were observed directly in the food grooves. The accessory tube feet are in bands leading toward the food grooves. Fellaster is so similar to Arachnoides that the description given here for Fellaster probably applies equally to Arachnoides. Complex Lateral Water Vessels The genera of the suborder Scutellina have complex schemes of lateral water vessel dis- Figure 12. Echinarachnius pari/ill, with long sections of its food grooves on the perradial sutures, after Durham (1955). tribution. Four genera were represented in the material studied. Echinarachnius has basically the same scheme as the other three studied but it is poorly developed. t seems transitional between the simpler schemes of Clypeaster and Fellaster, and the complex schemes of Dendraster, Encope, and Mellita. Echinarachnius has a major portion of each food groove on a perradial suture (Fig. 12). Branching only occurs in the region near the ambitus. Dendraster, Encope, and Mellita have complex polyfurating food grooves (Fig. 13). There are accessory tube feet within the food grooves throughout their length in the four genera. Since Echinarachnius seems to possess the most primitive scheme of the four genera it is described first. nternally each radial water vessel descends from the water vascular ring to an elevated stereom structure (perradial septum) on the perradial suture between two basicoronal ambulacral plates. Beyond the slope of the perradial septum each radial water vessel is supported on a slight ridge which extends along the perradial suture. There are pits au along the sides of each perradial septum and also along the sides of each perradial ridge. These pits provide access to the perradial food groove for lateral water vessels and ampullae serving accessory tube feet. Many lateral water vessels branch from a radial water vessel to serve the accessory tube feet of each plate. n the central cavity

12 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 411 Figure 13. Ellcopl' michelilli, with complex polyfurcating food grooves. Mellila and Leodia have similar food grooves, after Phelan (1972). of the test (area without internal supports and macrocanals) the lateral water vessels tend to curve toward the peristome (Figs. 14, 15) and extend about halfway across each plate toward the adradial suture. Branching occurs, but most of it is limited to short side branches on long vessels. The vessel nearest the adapical suture of a first post-basicoronal plate is described as an example. The long lateral vessel extends at an angle toward the peristome. t has numerous short branches on the adapical side of the vessel. Each of these short branches serves only two, three, or four accessory tube feet/ampullae. Ampullae may be attached to either side of a lateral water vessel. Throughout the plate many of these short branches from long lateral vessels lead toward the adapical, adoral, and adradial sutures (Figs. 14, 15). This seems to be a slight development of the scheme so highly developed in Dendraster, Encope, and Mellita. The lateral water vessels near the ambitus (area where food grooves branch) tend to be straighter, branch, and lie across entire plates. Some vessels extend into interambulacral plates mm ---- Figure 14. Echillarac/llius parma, adapicalfadradial portion of first post-basicoronal ambulacral plate showing position of lateral water vessels (wv) and accessory pores (a). Radial water vessel is at right (r). Dotted lines indicate plate sutures. New lateral water vessels, ampullae, and tube feet are added to plates as they grow, but the process by which they are added is not fully understood. Dendraster has polyfurcating food grooves (Fig. 16). A short portion of each lies on a perradial suture near the peristome. A main branch of each food groove extends down the middle of a column of ambulacral plates (through successive growth centers). There are many side tributary grooves and additional branches especially near the ambitus of the test. The food grooves are most devoloped in ambulacra and V, least developed in. Food grooves of the posterior region extend onto the adapical surface. nternally each radial water vessel descends from the water vascular ring to a perradial

13 412 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, mm f l ~, Figure 15. Echinarachnius parma, adoral/adradial corner of a second post-basicoronal ambulacral plate showing distribution of lateral water vessels (lwv) and accessory pores (a). Radial water vessel (r) at left. Dotted lines indicate plate sutures. Figure 16. Dendraster excentricus with differentially developed polyfurcating food grooves. The anterior food grooves are poorly developed. The posterior food grooves are highly developed. Many of the posterior grooves extend onto the adapical surface. septum as in Echinarachnius. Beyond the slope of the perradial septum each radial water vessel lies in a perradial groove. There are many pits on each side along the base of each perradial septum. These provide access to tube feet in the perradial portion of the food grooves. The portion of a perradial suture not occupied by a food groove has few or no accessory tube feet. This is also the condition in Encope (Durham, 1966b:456, fig. 341 b). nternally, lateral water vessels on postbasicoronal plates lie in open channels or microcanals, depending upon the degree of stereom development. Lateral vessels leave the radial water vessel at nearly right angles and extend to a point directly above a food groove (Fig. 17). At this point the lateral water vessels turn and lead toward the adradial suture (suture between ambulacral and interambulacral plates). On the abradial side of the plate (near the adradial suture) the lateral vessels branch extensively. At the adoral and adapical ends of a plate there is a slight difference in distribution. The lateral water vessels send branches to- ward the adoral and adapical plate sutures respectively and continue branching until they terminate. The result of this scheme of distribution is that lateral water vessels are always extending toward the growing edges of the plate and continually adding ampullae and tube feet. Water vessels extending from ambulacral plates onto interambulacral plates especially in the posterior regions repeat a modified form of this distribution scheme on interambulacral plates. Accessory tube foot ampullae are very close to the lateral water vessels. They are attached to either side of these vessels. Work is progressing on Dendraster to learn how lateral water vessels and accessory tube feet are added to plates as they expand with growth. Echinoids such as Encope, Mellita, and Leodia with notches or lunules in the ambulacra have an obstacle to overcome in serving accessory tube feet closer to the ambitus than the inner limits of notches or lunules. The arc in a radial water vessel between its adoral and adapical portions lies internally to the inner limits of an ambulacral notch or lunule.

14 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS mm Figure 17. Dendraster excentricus, view of internal surface of test at adapicaljadradial corner of a first post-basicoronal ambulacral plate and the adoral/ad radial portion of a secondpost-basicoronal ambulacral plate showing pattern of open channels (oc) which provide passageways for lateral water vessels. More than one vessel is present in some channels. Each vessel branches extensively. The ends of vessels lead toward the growing edges of the plates. Some channels are covered or partially covered. Radial water vessel (r) is at left. Dashed lines indicate position of plate sutures. n Leodia the area of the adoral surface beyond the inner ends of the ambulacral lunules is greater than the central area (Fig. 18). Lateral water vessels serving the areas beyond the inner limits of notches and lunules commonly cross several plates to serve accessory tube feet remote from the radial water vessels in genera such as Leodia, Encope, and Mellita. Lateral water vessel distribution within plates adjacent to the radial water vessel is very similar to that of Dendraster (Fig. 19). The following description is based on dis- Figure 18. Leodia sexiesperforata (Leske), outline of test and lunule positions, (A) area where radial water vessels are in contact with ambulacral plates, (B) larger area remote from radial water vessels where many ambulacral and interambulacral plates are served by lateral water vessels which branch from each radial water vessel at the inner ends of ambulacrallunules. sections of Encope but with minor changes applies also to Leodia and Mellita. nternally near the peristome each radial water vessel descends into the microcanal network of the adoral plates. There is a short elongate opening in the stereom above each radial vessel near the peristome. This opening does not extend beyond the tips of the lantern wings. The stereom between microcanals and the exterior surface is very thin, therefore the connecting vessels between the ampullae and the tube feet are quite short. The stereom is so thin in Encope michelini that where microcanals cross food grooves there is actually a little hump in the interior of the microcanals. This permits easy service to an abundance of accessory tube feet within the food grooves. The main branches of the food grooves and many tributary grooves extend into the region beyond the inner ends of ambulacral notches and lunules. nternally from the inner end of each ambulacral notch or lunule, very broad lateral water vessels branch from a radial water vessel. These vessels pass through microcanals commonly crossing

15 414 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, 1977 r 5 mm 5 mm Figure 19. Encope michelini, microcanal pattern within an adoral ambulacral plate. Radial water vessel (r) at left. More than one lateral water vessel commonly occupies each microcanal (me). Lateral water vessels lead toward growth center of the plate, turn and lead toward growing edges of plate. Water vessels branch profusely. Rotulina several plates, branch into numerous other microcanals, and serve accessory tube feet in the ambital region beyond the inner ends of notches and lunules (Fig. 20). Water vessel distribution within plates of this region is basically the same as that described for Dendraster with water vessels leading toward the growing edges of each plate. Accessory tube feet and vessels are added as plates grow. The microcanals may serve additional functions other than as passageways for lateral water vessels as some microcanals extend into areas not supplied with water vessels. The single specimen of Heliophora orbiculus (Linnaeus) available for study was a dry test completely lacking tissue. t was inadequate for any determination of the water vessel distribution in the suborder Rotulina. The food grooves are polyfurcating with a main branch extending along the growth centers of each column of ambulacral and \ --- \ Figure 20. Encope michelini, microcanal network which serves long lateral water vessels whieh leave radial vessel (r) at inner edge of ambital notch. Microcanals which serve only individual plates omitted from drawing. interambulacral plates. No small tributary grooves were observed leading into the main branches. The food grooves extend onto the finger-like extensions along the posterior ambitus formed by deep notching along perradial, ad radial, and interradial sutures. Throughout the food grooves in ambulacra and interambulacra there are numerous pores indicating they are supplied with accessory tube feet. RELATONSHP BETWEEN WATER VESSELS, ACCESSORY TUBE FEET, PETALOD PLATES, AND FOOD GROOVES OF THE ADAPCAL SURFACE Accessory tube feet of the margin and adapical surface help collect food as do those of the adoral surface. They also lift sand, shell fragments, algae, and other material from the substrate and working in conjunction with spines pass it over the adapical surface. The particles may be for covering or the product of burrowing. Kier produced research cine films in both normal and time lapse photography showing this activity. Kier and Grant (l965:pl. 4-7) show several species carrying a wide variety of material including dead echinoid tests and show several species covering themselves with sand in the process of burrowing. Nichols

16 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 415 lmm.. rp Figure 21. Echinaracl/llius parma, internal view of the adapical end of a petal showing accessory pores (a). Note that no accessory pores are located in the poriferous zones (pz) of the respiratory pores (rp). Petal is composed of simple primary plates. (1959: ) describes the covering and burrowing activity of Echinocyamus pusillus. Poriferous Zone No accessory tube feet were observed in the poriferous zones of the respiratory tube feet (Figs. 2], 24B) have looked for this condition and have not observed it on any specimen studied. Petaloid Plates There are two basic petaloid plate arrangementsin the c1ypeasteroids. One is a column of simple primary plates in each half ambulacrum (Fig. 21). The other has a column of primary plates in each half ambulacrum which are narrow in the region of the respiratory tube feet. Here demiplates alternate with primary plates (Fig. 24B). There are more complex arrangements of demiplates in some fossil clypeasteroids. For details of these see Durham (1955 :fig. 27a-c). Petals with Primary Plates only Echinarachnius has this plate arrangement and is used for the first description. The lateral water vessels, pores for accessory tube feet, and respiratory tube feet lie along the sutures between ambulacral plates. The lateral water vessels and accessory pores arc slightly to the side of some sutures or lie across a suture at a slight angle. There is only one lateral water vessel along each suture and it serves both accessory tube feet and a respiratory tube foot (Fig. 21). The youngest plates observed accommodating accessory tube feet were approximately the sixth to eighth from the terminal plate. All plates adoral to this point had accessory tube feet. have observed living populations of Echinarachnius parma while diving off Cape May, New Jersey. They did not burrow or cover themselves. The accessory tube feet of the petals probably primarily serve in collecting food. There was little or no sand in the intestinal tract. MacBride (]909:545) reported that the accessory tube feet of E. parma are present in all plates, ambulacral and interambulacral, on the adapical surface. This is inaccurate. E. parma has accessory tube feet in both ambulacral and interambulacral plates at and near the ambitus on the adapical surface similar to many c1ypeasteroids. There are no accessory tube feet in the interambulacral plates between the petals of E. parma. One of his figures of E. parma (MacBride, 1909: fig. 242 a,b) is misidentified. The figured specimen is Dendraster excentricus. MacBride (1909:546) observed E. parma in comparatively shallow water on a sandy bottom and described them as nearly but not quite buried in sand. Stanley and James (1971 :p. 1:figs. b, c; p. 2: figs. a-c; p. 3: figs. d, e) photographed living populations of E. parma on the ocean floor. Most of the sand dollars are on the surface of the substrate. n several photographs a few are partially buried as MacBride reported.

17 416 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, mm Figure 22. Dendraster excentricus, internal view of posterior petal (). Radial water vessel depicted by dotted zone (rr) along the perradial suture. Pseudopores (c) leading to cavities for accessory tube foot ampullae adjacent to the radial water vessel (r). Outer pairs of pores are for respiratory tube feet (rp). Dendraster excentricus (Eschscholtz) of the Pacific coast also ingests little sand. Accessory tube foot distribution is very closely related to food groove development on the adoral surface. The anterior petal () has very few accessory tube feet. Most of these are near the distal end of the petal. The anterior adoral food grooves are poorly developed. The posterior paired petals ( and V) have an abundance of accessory tube feet throughout their length. The posterior food grooves are highly developed and extend onto the adapical surface. A series of glass]y tubercles from within the distal end of each petal leads down each column of ambu]acra] plates and enters a food groove. Each series of glassy tubercles has a band of accessory tube feet in the same pathway. The anterior paired petals ( and V) also reflect the poorly developed anterior and well developed posterior adoral food grooves. The anterior half of these petals (lb and Va) have fewer accessory tube feet than the posterior half (lla and Vb). think this distribution in relation to food groove development strongly reflects the food gathering function of the accessory tube feet. (See Durham, 1966:269.) have observed live specimens of Dendraster and only the anterior portion was buried in the substrate. n Dendraster the accessory tube feet seem to function more for feeding than Figure 23. D. excelltricus, cross section along suture between plates of one half petal showing pselldopore and cavity (c) for accessory tube feet, and respiratory pores (rp). Note partitions in outer (abradial) pore. burrowing. The accessory tube feet collect particles of organic material suspended in the water and also pick up food particles from the substrate. The collected material is passed to the mouth along the food grooves. The interior surface of petals and V appears to have a triple series of pores in each column of ambulacral plates. Only the two outer series are pores for the respiratory tube feet. The innermost series are pseudopores. They lead to cavities in the plates to accommodate the ampullae of accessory tube feet (Figs. la, 22, 23). These cavities are less developed on petals and V and virtually absent on petal. The plates with cavities for accessory tube foot ampullae have a different system of lateral water vessel distribution than occurs in Echinarachnius. The accessory tube feet/ampullae are served by separate vessels from those serving respiratory tube feet. Two lateral water vesse]sleave the radial water vessel at approximately the same point. Some of these connections appear to be branching of a single vessel very near the radial water vessel. One lateral water vessel leads along the suture to the ampulla of a respiratory tube foot. The other vessel descends immediately into the cavity to serve the ampullae of the accessory tube feet. Petals with Primary and Demiplates The lateral water vessels do not lie along plate sutures in clypeasteroids with altern at-

18 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 417 rp 101m f------l ~. ~ ~r p ~" ~. :\ ~ B Figure 24. Clypeasler subdepressus, (A) internal view of the adapical end of a petal showing, by the course of accessory tube foot pores (a), how some lateral water vessels lie across plate sutures. The junctions of the lateral water vessels with a radial water vessel are on plates more remote from the apical system than the plates where the distal ends of the lateral vessels connect to respiratory tube foot ampullae; respiratory tube foot/ampullae pores (rp). Radial water vesscllies along perradial suture (s). (B) nternal view of mid-petal area showing, by course of accessory tube foot pores, the position of the two lateral water vessels on each plate. Accessory tube foot pores do not extend into the poriferous zone (pz) of the respiratory tube feet. Petals arc composed of primary and demiplates. ing primary and demiplates. Clypeaster and Fellaster have this alternation of primary and demiplates. Two lateral water vessels lie on the internal surface of each primary plate. Each terminates at the inner pore of a respiratory tube foot in contact with that plate (Figs. 24B, 26, 27). Two respiratory tube feet are in contact with each primary plate. At the middle and distal ends of the petals some primary plates have an additional lateral water vessel which serves only accessory tube feet. The lateral vessels serving respiratory tube feet also serve accessory tube feet. The ampullae for the accessory tube feet are attached to the adapical side of the lateral water vessels of Clypeaster. t is undetermined on Fellaster. As plates grow new ampullae and accessory tube feet are added to the lateral water vessels. On Clypeaster subdepressus (Gray) and the number of ampullae on a single vessel was observed to be nearly equal to the number of growth lines on the plate served. nternally in Clypeaster subdepressus 4 mm Figure 25. Clypeaster subdepressus, exterior surface of the adapical portion of a petal. Large black pores are respiratory pores (rp). Concentric circles are sunken tubercles and areoles. The small ring around the adapical portion of each accessory pore (ap) is a depression formed of small pits in the stereom to which the accessory tube foot is attached. some petals examined had lateral water vessels lying across sutures between primary plates of a column. At the proximal end of a petal with this condition, the connections of lateral water vessels to the radial water vessel is at plates more remote from the apical system than the plates to which these vessels connect to respiratory ampullae. These vessels serve accessory ampullae on more than one plate. Figure 24A is a drawing of plates with this condition. The tissue was removed from the stereom prior to photographing the plates. The course of the lateral water vessels was along the rows of accessory pores. n the large mid-petal region some lateral water vessels lie across plate sutures. The two or three vessels on each primary plate commonly lie almost parallel to the sutures between plates of the column (adapical and adoral plate sutures). Figure 24B is a drawing of plates on which the accessory pores lie in two rows nearly parallel to the adapicaljadoral plate sutures. The photograph for the drawing was made after the tissue was removed from the stereom.

19 418 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, mm l ap !.. -~. "- - ~ ll "'- -, -,- - -,--. ' Figure 26. Fellasler zelandiae, internal view of a portion of a petal showing the position of the lateral vessels by position of accessory pores (ap). Pores are more numerous than can be shown at this low magnification. Petals are composed of primary and demiplates. Accessory pores are in rows on the interior surface of the petals. The pores do not pass straight through the stereom to the exterior but pass through at various angles. The result is a random arrangement of pores on the exterior surface (Fig. 25). The lateral water vessels of Pel/aster also align parallel to the sutures between ambulacral plates. At the adapical end of the petal there are two lateral vessels on each primary plate. There are a few plates in the middle of the petals with an additional vessel. At the distal end of a petal, most primary plates have three lateral water vessels-two serving respiratory and accessory tube feet and one serving only accessory tube feet. The accessory pores on the interior surface of each primary plate are in alignment with the lateral water vessels (Fig. 26). The pores realign in passing through each plate. On the exterior surface of each primary plate there is a series of diagonal rows of accessory pores. The diagonal rows of adjacent plates align more or less giving the appearance of continuous rows. The diagonal bands of accessory tube foot pores cross several plates in an adoraljadradial direction and terminate at the edge of a perradial food groove (Figs. 27, 28). Each food groove extends from Figure 27. Fellasler zelandiae, semi-block diagram of section of a petal. Dashes on surface indicate position of adapical and adoral plate sutures (ps). Food groove (not shown) is toward the left. Circles represent tubercles (t). Water vessels (less ampullae) are drawn in at right to indicate their position parallel to plate sutures. Each water vessel serves half of the accessory tube feet of each plate (except on plates with three vessels). The pores on the internal surface of the plates are aligned along the water vessels. The oblique lineation on surface results from variable inclination of pores passing through to the exterior surface of plates. The scale applies only to the external surface. near the apical system to the peristome. There are no tube feet in the food grooves. The diagonal bands of accessory tube feet are separated by diagonal bands of large and small spines and tubercles. This condition results in the so-called "combed" areas of Fellaster and Arachnoides. EXPANSON OF AMBULACRAL PLATES AND NTERRUPTED NTERAMBULACRAL COLUMNS Clypeasteroid ambulacral plates commonly widen near the distal end of the petaloid area. The ambulacral columns are commonly wider than the interambulacral columns at the ambitus (Fig. 29A). On the adoral side expanded post-basicoronaj ambulacral plates commonly interrupt the

20 PHELAN: WATER VASCULAR SYSTEM OF ECHTNODS 419 lmm ".Uo 0-.. '-'0 : ps OU"-"OU 0 0""0 V 0 ". 0 ) ".',00 ; 0 "0 0 0,; O "; 0 :, oq,,'"?ro 00~::~o(----~~-~-.. ~-do-o- O..~?o 0,00! g600000~' ;O"O~ -~~;.~;~oo----\,0 0,o~c;:~'O:'~8 ( " ) n...,'..~o ' 0941 ~.,, ,,0'9 0,, " "" fg {l'j"r\ r ap Figure 28. Fe/laster zealandiae, small section of the perradial portion of the petal, accessory tube foot pores lead in an adoraljadradial direction toward the food groove (fg). Dotted lines indicate position of plate sutures (ps). Note the absence of accessory tube foot pores in the food groove. columns of interambulacral plates (Fig. 29B). For additional information see Durham (1955:figs. 17,21-25). believe the great expansion of the ambulacra is related to the increased food gathering capacity afforded by a broad distribution of accessory tube feet. H the clypeasteroids evolved from the juvenile stage of a cassiduloid (discussed later in this paper) the tendency for expanded ambulacral plates may have preceded the development of true accessory tube feet. The phyllodes of cassiduloids are formed by an expansion of post-basicoronal plates which support specialized feeding tube feet. Late in the Cretaceous these tube feet evolved from the two pore to single pore forms (Kier, 1962:chart 2). Accessory tube feet are also served by a single pore. Cassiduloid tube feet between the phyllodes and the petals also evolved into the single pore form. The large areas served by accessory tube feet on clypeasteroids are truly phyllodes in my opinion. They fit the broad definition of the term as used by Kier (1974: 20). The accessory tube feet are so small that these clypeasteroid phyllodes are not as readily recognized as they are on other echinoids. Figure 29. Dendraster excenlricus, expansion of ambulacra (stippled) beyond the petals, (A) adapical view, (B) adoral view. Note interrupted interambulacrum 5. ndividual plates are not shown. FEATURES SUPPORTNG THE CASSDULOD ANCESTRY OF CLYPEASTERODS Kier (1974:88) has raised doubts that any clypeasteroid has yet been found in the Cretaceous. The clypeasteroids evolved rapidly once established. During the middle Eocene most genera had some simple unsophisticated test characters such as a single pore in the madreporite, poorly defined petals, simple round non-conjugate pore pairs, slightly developed or no internal supports, ovid test, and a lack of food grooves. Genera with some of these simple characters are Fibularia, Echinocyamus, Lenita, Leniechinus, and Pentedium. Many of these simple characters are common in juveniles of genera with highly sophisticated test characters. Some middle Eocene genera had developed a relatively high degree of sophistication. Periarchus had pores distributed throughout the madreporite, food grooves, internal supports, and petal pore pairs with elongate outer pores. The clypeasteroid record is poor earlier than the middle Eocene. believe the simple test characters of many middle Eocene species support the hypothesis that clypeasteroids evolved from the juvenile stage of their ancestor. Nichols (personal communication, 1976) stated that small size does not necessarily suggest neotenic derivation from a juvenile. t is not genetically difficult to alter the size of

21 420 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, 1977 an animal, whatever its origin. Most middle Eocene clypeasteroids were relatively small. Whether this is a factor of their neotenic derivation is not known. The fibulariids are apparently the most primitive of the c1ypeasteroids based on test characters. One of these is a very juvenile level of madreporite development. t is common for sexually mature fossil and Recent fibulariids to have a single madreporite pore. Only one pore is present in the madreporite in the earliest stage of development in echinoids (Hyman, 1955:470). This is the hydropore from the larval stage. This does not necessarily indicate a cassiduloid ancestor but it is a strong juvenile feature representing a level at or near metamorphosis. Some interesting changes were taking place within the cassiduloids as the Cretaceous ended. Many of these changes resulted in features common to cassiduloids and clypeasteroids. The cassiduloids developed a monobasal apical system (Kier, 1962: chart 1). An exception is the Apatopygidae (Kier, 1974:33, Fig. 28c). Clypeasteroids also have a monobasal apical system. The double pore tube feet beyond the petals of cassiduloids evolved into the single pore form (Kier, 1962, chart 1). The accessory tube feet of clypeasteroids each have a single pore. The respiratory tube foot pores shifted from within plates to between plates (on the suture) on cassiduloids. Clypeasteroids have their respiratory tube foot pores similarly located between plates. Cassiduloids developed single pore buccal tube feet, one on each basicoronal ambulacral plate (Kier, 1962: chart 2). Many genera of clypeasteroids have a single pore buccal tube foot on each basicoronal ambulacral plate. Recent immature cassiduloids a few millimeters in length are known to have a low profile lantern with large wings unlike the erect lantern of regular and holectypoid echinoids. The lantern of immature cassiduloids is very similar to the lantern of oligopygoids and clypeasteroids (Kier, 1974:51-56, figs. 56a-d). The oligopygoids probably also evolved from the immature stage of a cassiduloid. The juvenile cassiduloid has the same basicoronal plate arrangement and lantern support structure as the oligopygoids (Kier, 1974:59-61). The absence of compasses in the clypeasteroids has been explained as due to the absence of gills and a nonseparation of the lantern from the main coelomic cavity of the test (Nichols, 1962:69) (Kier, 1974: 56). Another possibility is that the clypeasteroids evolved from the juvenile stage of a cassiduloid. Compasses never fully develop in cassiduloids. All ossicles of the lantern except the compasses are formed before an echinoid passes through metamorphosis (Hyman, 1955:496). t could be that the absence of compasses and presence of a lantern in the clypeasteroids and oigopygoids is related to the juvenile level of development at which these orders evolved from their respective cassiduloid ancestors. Compasses were present in echinoids early in the Paleozoic Era prior to the development of echinoid gills. The lantern of a clypeasteroid is not enclosed within a loose fitting peripharyngeal membrane as in the regular echinoids, but close examination reveals the lantern is effectively sheathed in close fitting membranes. CONCLUSONS The ampullae of pore pair supported tube feet have many features in common, even comparing the respiratory tube foot ampullae of clypeasteroids with the ampullae of regular urchins. Both ampullae are flattened. Fluid constantly circulates between tube feet and ampullae. Both ampullae have facilities for distribution of the current, and very thin walls. Specialized bulb shaped ampullae somewhat like those of asteroids are found in echinoids where the pore pairs have evolved into a single pore. Examples are the ten buccal tube feet or peristomials of regulars and the buccal tube feet and accessory tube feet of clypeasteroids. The complex water vessel network serving

22 PHELAN: WATER VASCULAR SYSTEM OF ECHNODS 421 accessory tube feet tends to parallel development of the food grooves. This is evident on Dendraster where the anterior food grooves are poorly developed and those of the posterior are highly developed. The fibulariids, which lack food grooves, have simple lobes on the radial water vessels to serve accessory tube feet. Laganids have perradial food grooves and accessory tube feet are served by bands of ampullae along the sides of the radial water vessels and blind ending side spurs. Clypeaster and Fellaster have perradial food grooves and simple lateral water vessels, many for each ambulacral plate. Echinarachnius has a long segment of each food groove on a perradial suture but many tributary branches near the ambitus of the test. There are many lateral water vessels for each plate, some with short branches which tend to lead toward the growing edges of the plate. Encope, Me/lita, and Dendraster have polyfurcating food grooves and many lateral water vessels for each plate. The lateral water vessels repeatedly branch leading toward the growing edges of the plates and even send long vessels into interambulacral plates. n clypeasteroids with true lateral water vessels, new vessels are added, older vessels lengthened, and additional accessory tube feet/ampullae formed as the plates grow. The lateral water vessels that serve respiratory tube feet commonly also serve accessory tube feet. Some plates within the petals have lateral vessels which serve only accessory tube feet. Lateral water vessels in the petals commonly are parallel to adapicalj adoral plate sutures regardless of the pattern of. pores on the exterior surface (e.g. F ellaster). The accessory tube foot ampullae are within cavities in the plates in the petals of Dendraster. No accessory tube feet were observed within the poriferous zones of the respiratory tube feet. The great lateral expansion of ambulacral plates and adoral interruption of interambulacral columns is believed closely related to the development of the accessory tube foot system. They provide a broad food gathering capability. These expanded areas are considered phyllodes and homologous to those of the cassiduloids. They possibly have not been previously recognized as phyllodes due to the near microscopic size of the structures of the test that accompany the abundant minute accessory tube feet. Numerous features are common to the cassiduloids and clypeasteroids, strongly suggesting that the cassiduloids are ancestral to clypeasteroids. Many middle Eocene clypeasteroids have juvenile characters possibly supporting the hypothesis that the clypeasteroids evolved from sexually mature juvenile cassiduloids. The absence of compasses and possession of a lantern in clypeasteroids are also suggested as juvenile features supporting neotonic derivation of the clypeasteroids from the cassiduloids. ACKNOWLEDGMENTS thank Wyatt Durham, David Nichols, Porter M. Kier, and Carol Wagner Allison for critically reading the manuscript and offering excellent suggestions; the staff of the Oregon nstitute of Marine Biology, Paul P. Rudy and Robert C. Terwilliger for providing laboratory facilities, Jean Hanna and Kim Baxter for assistance in laboratory techniques; POrter M. Kier for providing encour"agement and information. Specimens for study were provided by Maureen Downey, Wyatt Durham, Porter M. Kier, Klaus Ruetzler, J. Ross Wilcox, and the staff of the Oregon nstitute of Marine Biology. thank my wife, Kathleen Phelan for typing the manuscript. LTERATURE CTED Durham, 1. W Classification of clypeasteroid echinoids. California Univ., Pub\. Geol. Sci. 31: , ps. 3, 4, 38 text-figs a. Anatomy. Pages in R. C. Moore, ed. Treatise on paleontology. Kansas Univ. Press and Geo\. Soc. America, Lawrence, Kansas, pt. U Echinodermata b. Clypeasteroids. Pages ill R. C. Moore, ed. Treatise on paleontology. Kansas Univ. Press and Geol. Soc. America, Lawrence, Kansas, pt. U Echinodermata 3. Hyman, L. H The invertebrates. Echinodermata 4. McGraw-Hill Book Co., New York, 763 pp., 280 figs. Kier, P. M Revision of the cassiduloid echinoids. Smithson. Misc. Col. 144 (3): 262 pp., 44 ps., 184 text-figs.

23 422 BULLETN OF MARNE SCENCE, VOL. 27, NO.3, Echinoids from the Middle Eocene Lake City Formation of Georgia. Smithson. Misc. Coli. 153(2): 45 pp., 10 ps., 44 test-figs Evolutionary trends and their functional significance in the Post-Paleozoic echinoids. Paleont. Soc., Mem. 5. Jour. Paleontology, 48(3 supp.): 96 pp., 78 figs. ---, and R. E. Grant Echinoid distribution and habits, Key Largo Coral Reef Preserve, Florida. Smithson. Misc. Col. 149(6): 68 pp., 16 pis., 15 text-figs. MacBride, E. W Echinodermata. Pages in S. F. Harmer and A. E. Shipley, eds. Cambridge Natural History, London. Macmillan and Co. Ltd. Nichols, D The histology and activities of the tube-feet of Echinocyamas pasillas. Quart. Jour. of Microscopical Sci., 100: , 8 figs A comparative histological study of the tube-feet of two regular echinoids. Quart. Jour. Microscopical Sci. 102: , 12 figs Echinoderms. Hutchinson Univ. Library (London). 200 pp., 26 figs. Phelan, T. F Comments on the echinoid genus Encope, and a new subgenus. Proc. Bio. Soc. Washington. 85: , 11 figs. Stanley, D. J., and N. P. James Distribution of Echinai'Qcllllius parma (Lamarck) and associated fauna on Sable sland Bank, southeast Canada. Smithson. Contr. Earth Sci. No. 6: 24 pp., 6 p]s., 8 text figs. DATE ACCEPTED: July 21, ADDRESS: Oregon nstitute of Marine Biology, Charleston, Oregon