Mollusca Type Study Pila (Apple Snail) Part - B. Topic No. & Title: Topic 8 Mollusca - Type Study, Torsion & Detorsion in Gastropods

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Mollusca Type Study Pila (Apple Snail) Part - B [Academic Script] Subject: Course: Paper No. & Title: Zoology B.Sc. 1 st Year Z-101B Animal Diversity-I Topic No.

1 Mollusca Type Study Pila (Apple Snail) Part - B [Academic Script] Subject: Course: Paper No. & Title: Zoology B.Sc. 1 st Year Z-101B Animal Diversity-I Topic No. & Title: Topic 8 Mollusca - Type Study, Torsion & Detorsion in Gastropods Lecture Title: Part - B Mollusca - Type Study Pila (Apple Snail).

2 Academic Script 1.Introduction HABIT AND HABITAT Pila globosa or the apple snail is one of the largest freshwater molluscs. It is commonly found in fresh water ponds, pools, tanks, lakes, marshes, rice fields and sometimes even in streams and rivers even in brackish waters of low salinity. The animal creeps very slowly by its ventral muscular foot, covering about five cm per minute. During the rainy seasons Pila comes out of the ponds and makes long terrestrial tours. SYSTEMATIC POSITION Phylum: Molluscs Class: Gastropoda Subclass: Prosobranchia Order: Megagastropoda Genus: Pila Species: globosa 2. MORPHOLOGY The shell of Pila, as in other Gastropoda, is univalve but coiled around a central axis in a right-handed spiral. The top of shell is the apex which is formed first and growth of shell takes place from it. Below the apex is a spire consisting of several larger whorls or coils followed by penultimate whorl and largest body whorl. The lines between the whorls are called sutures.

3 The body whorl has a large mouth or opening, the margin of the mouth is called as peristome. In the center of the shell runs a vertical axis or columella. The columella is hollow and its opening to the exterior is known as umbilicus. Fitting into the mouth of the shell is a calcareous operculum. The body consists of head, foot and a visceral mass. A collumellar muscle arises from the foot and is inserted in the collumella, it attaches the body to the shell and it withdraws the animal inside and closes the operculum. The mantle also referred to as palium covers the visceral mass and it forms a hood over the animal when it is withdrawn. COELOM The coelom is reduced to unpaired cavities of pericardium, kidney and gonad. The renal and pericardial cavities communicate, but the cavity of gonad is unconnected. The visceral organs are surrounded by means of sinuses or space containing blood. These blood filled spaces constitute the haemocoel. 3. DIGESTIVE SYSTEM The digestive system of Pila comprises a tubular alimentary canal, a pair of salivary glands and a large digestive gland. Alimentary Canal The alimentary canal is distinguished into three regions:

4 (1) the foregut or stomodaeum including the buccal mass and oesophagus, (2) the midgut or mesenteron consisting of stomach and intestine (3) the hind gut or proctodaeum comprising the rectum. Foregut. The foregut includes the mouth, buccal mass and oesophagus. The mouth is a narrow vertical slit situated at the end of snout. There are no true lips but the plicate edges serve as secondary lips. The mouth leads into a large cavity of buccal mass or pharynx having thick walls with several sets of muscles. The anterior part of the buccal mass is vestibule. The jaws bear muscles and their anterior edges have teeth like projections for cutting up vegetable food. Behind the jaws is a large buccal cavity. On the floor of the buccal cavity is a large elevation called odontophore. Above and behind the odontophore is a bag like radular sac which is a diverticulum of a buccal cavity. The radular sac has transverse rows of cells called odontoblasts. Inside the radular sac is a radula which is characteristic of Mollusca. The radula is made of many transverse rows of horny teeth. The buccal mass leads into a long narrow oesophagus. From near the origin of the oesophagus arise a pair of round, whitish oesophageal pouches. They arise by short ducts and lie below the salivary glands.

5 Midgut. The midgut includes the stomach and intestine. The stomach is red in color and is situated on the lower part of the visceral mass just below the pericardium. It is a large sac and bent on itself to form a U -tube, one limb of which receives the oesophagus and the other leads into the intestine. The end which receives the oesophagus is called the posterior or cardiac chamber, while other end is called the pyloric chamber. Hindgut. The rectum or terminal part of the alimentary canal is a thick walled tube. It enters the mantle cavity and passes downwards to open by an anus on the rightside of the head. The digestive glands include the salivary glands and the liver. There are two salivary glands situated one on each side of the oesophagus. The liver is black in color. 4. LOCOMOTION Pila moves very slowly by creeping on the substratum by its foot. During movement the foot is protruded through the opening of the shell and its flat sole helps in the process. The extension of the foot is caused by sudden influx of blood into the foot. The glands present in the foot produce slimy secretion that helps the animal to glide on dry surface. During locomotion the wave likes contractions on its surface the produced by the contractions of the vertical muscles. The contractions of the transverse muscles drive the blood forward which causes the extension of the foot in front.

6 5.RESPIRATORY SYSTEM Pila leads an amphibious life and, therefore, it exhibits dual modes of respiration, i.e., aquatic and aerial. For these, it possesses a ctenidium or gill (for aquatic respiration) and pulmonary sac or lung (for aerial respiration). The respiratory organs consist of a single ctenidium or gill, a pulmonary sac or lung and a pair of nuchal lobes. Ctenidium or gill. The ctenidium or gill is the organ of aquatic respiration. The ctenidium is situated on the dorso-lateral wall of the branchial chamber of the mantle cavity. It is composed of a long series of thin triangular leaflets or lamellae, lying parallel to each other, which are attached to the mantle wall by their broad bases but have their apices hanging free in the branchial chamber. These lamellae are arranged in single row running parallel to one another along the ctenidial axis of the gill. This type of ctenidium is known as monopectinate type. There are numerous gill lamellae. The basal end of each gill lamella is attached to the pallial epithelium and other end hangs freely to the mantle cavity. The gill filaments are lined by ciliated epithelium towards the free end and the basal portion is composed of non-ciliated epithelial cell and few glandular cells. Pulmonary sac. The pulmonary sac or lung is a closed cavity like a bag which hangs from the dorsal wall of the mantle in the pulmonary chamber. The dorsal wall of the pulmonary sac is densely pigmented while the ventral wall is creamy white. The walls of the sac are highly vascular, i.e., richly supplied with blood

7 vessels. The pulmonary sac communicates with the pulmonary chamber of the branchial cavity by an aperture known as the pneumotome. Nuchal lobes. The right and left nuchal lobes are fleshy and highly contractile processes of the mantle on either side of the head. They form elongated funnels or siphons during respiration for the entry and exit of water. 6.CIRCULATORY SYSTEM The circulatory system is open and consists of heart, arteries, veins and the sinuses and has attained great complexity because of its amphibious nature. The heart is situated on the left side of visceral mass very close to the posterior end of the ctenidium. The heart is enclosed in a thinwalled coelomic cavity called pericardium. The pericardium is situated between the pulmonary chamber and the posterior renal sac on the left side of the body whorl. It extends anteriorly upto the stomach and digestive gland and communicates with the posterior renal chamber by a renopericardial aperture. Heart consists of two chambers: an auricle and a ventricle. The auricle is thin walled, highly contractile and lies in the dorsal part of the pericardium. Ventricle is thick walled, spongy and muscular situated just below the auricle in the same vertical axis. The auricle receives oxygenated blood from the ctenidium and the pulmonary sac through the efferent ctenidial vein and the pulmonary vein. It also receives blood from the posterior renal chamber by efferent renal vein.

8 The lower end of the ventricle gives rise to an aorta which immediately divides into two branches: 1. An anterior cephalic aorta 2. Posterior visceral aorta. The cephalic aorta supplies blood to the head region. It is swollen into a thick walled contractile aortic ampulla at its base which helps in the propulsion of the blood. The cephalic aorta gives off three branches on its outer side to skin, oesophagus and mantle. The visceral aorta passes through the visceral mass and supplies blood to the visceral organs. It gives out: A small pericardial artery to the pericardium. A large and stout gastric artery to the stomach. Intestinal arteries to the intestine. A hepatic artery to the digestive gland and the gonads. Renal arteries to the anterior and posterior renal chamber. Finally the visceral aorta terminates into branches that supply the rectum and the anus. There are four blood sinuses in Pila: 1. A peri-visceral sinus surrounding the anterior part of the alimentary canal. It contains deoxygenated blood. Three channels carry blood from this sinus to the renal chamber and the pulmonary sac. 2. Peri-intestinal sinus surrounding the terminal part of the intestine and the genital duct. It collects blood from the digestive gland and the visceral organs.

9 3. Branchio-renal sinus lies outside the anterior renal chamber and leads into the afferent ctenidial vein. It receives blood from the roof of both the renal chambers. 4. Pulmonary sinus present in the wall of the pulmonary sac. It receives blood from the peri-visceral sinus. BLOOD Blood is colorless because of the absence of haemoglobin. It contains the respiratory pigment haemocyanin which is dissolved in blood plasma. COURSE OF CIRCULATION The cephalic and visceral aorta supplies blood to different parts of the body. The blood is then collected from various parts of the body in two main sinuses, the peri-visceral and peri-intestinal. From these sinuses, the blood passes either into the ctenidium, pulmonary sac or the kidney. During aerial respiration, the blood flows from the peri-visceral sinus into the pulmonary sac and after aeration comes to the auricle by the pulmonary vein. During aquatic respiration, the blood flows from the perivisceral sinus to the ctenidium and after aeration comes to the auricle by the efferent ctenidial vein. The blood from the peri- intestinal sinus takes two courses: it either goes to the anterior renal chamber from where it goes into the ctenidium where it is purified and finally through the efferent ctenidial vein enters the auricle, or from the peri-intestinal sinus, the blood goes to the anterior renal chamber and then into the posterior renal chamber to the

10 auricle. This blood is not aerated. Thus, the aerated and nonaerated blood gets mixed up in the auricle. This mixed blood then enters the ventricle and is distributed to the arteries. The renal chambers remove the excretory products from the blood. 7.EXCRETORY SYSTEM Excretion is performed by a large thick walled muscular renal organ or kidney which consists of an anterior chamber and a posterior chamber, the two communicating with each other by an aperture guarded by a valve. The renal organ communicates with the exterior on one hand and with the pericardial cavity on the other. ANTERIOR RENAL CHAMBER The anterior chamber is more or less ovoid, reddish in color, smaller than the posterior chamber, situated in front of the pericardium. It opens into the mantle cavity by an elongated aperture on the right side of the epitaenial ridge. On the other end it communicates with the posterior renal chamber through an internal opening. The internal cavity of the anterior chamber is reduced due to the presence of numerous lamellae projecting both from the roof and the floor. The lamellae are arranged on the floor on either side of a single median axis, the afferent renal sinus and also on the roof on either side of the median longitudinal axis, the efferent renal sinus. POSTERIOR RENAL CHAMBER The posterior renal chamber lies on the left side of the rectum, closely pressed against the pericardium and the

11 digestive gland. Its cavity communicates with the pericardium by the reno-pericardial aperture at one end while the other end communicates with the anterior renal chamber by an aperture. The posterior renal chamber is broad, somewhat hook shaped, brownish to grey in color. MECHANISM OF EXCRETION The walls of both the chambers are richly supplied with a network of blood vessels. The renal chambers separate the nitrogenous waste material from the blood. The excretory products are passed from the posterior renal chamber into the anterior renal chamber, from where it is discharged into the mantle cavity through the renal duct. The waste products are finally expelled out of the body from the mantle cavity through the right nuchal lobe along with the outgoing water current. The excretory fluid mainly contains ammonia, ammonium compounds, urea and uric acid. During the aquatic phase, Pila excretes out ammonia (ammonotelic) but during the terrestrial phase it excretes out uric acid (uricotelic). This is so because during the terrestrial phase it needs to conserve water. 8.NERVOUS SYSTEM The nervous system consists of series of paired ganglia with commissures and connectives. The term commissure is applied to a nerve cord connecting two similar ganglia, while connective is applied to a nerve cord connecting two dissimilar ganglia. A pair of cerebral ganglia lies anteriorly on the dorsolateral sides of the buccal mass, one on each side of the head. The two cerebral ganglia are connected by a thick

12 cerebral commissure running transversely above the buccal mass. Each cerebral ganglia gives off nerves that innervate the eye, the snout, statocyst and the tentacles of its side. A pair of buccal ganglia lies at the junction of the buccal mass and the oesophagus. They are connected to each other by a delicate transverse buccal commissure. Each buccal ganglion is also connected to the cerebral ganglion of its side by a cerebro- buccal connective, so that a nerve ring is formed anteriorly encircling the gut. The buccal ganglion gives off nerves to the buccal mass, radular sac, salivary glands, oesophagus and the oesophageal pouches. A pair of pleuro-pedal ganglionic mass is present on the ventro-lateral side of the buccal mass, one on the right side and the other on the left side. Each pleuro-pedal ganglionic mass is formed by the partial fusion of outer pleural and inner pedal ganglia which lie very close to one another and are separated only by a notch. The two pedal ganglia are connected by a broad pedal commissure lying under the buccal mass. The pedal ganglion gives out numerous nerves to the foot. The left pleural ganglion supplies the nerves to the body wall, mantle, osphradium, left nuchal lobe, columellar muscles, and anterior part of ctenidia while the right pleural ganglion innervates the partial wall, epitaenia, right nuchal lobe, copulatory organs, columellar muscles and the rectum. The cerebral ganglion of each side is connected to the pleuro-pedal ganglionic mass of its side by two connectives called the cerebro-pedal (connecting the cerebral and the inner pedal ganglion) and cerebro-pleural connective (connecting the cerebral and the outer pleural ganglion).

13 A very slender nerve called the infra intestinal nerve connects the pleural ganglion of the two sides. It forms a loop just behind the pedal commissure. An unpaired supra-intestinal ganglion lies a little behind the left pleuro-pedal ganglionic mass. The supra-intestinal ganglion is connected to the right pleuro-pedal ganglionic mass by a very fine oblique nerve lying just above the oesophagus called as the supra-intestinal nerve. The supraintestinal ganglion supplies nerves to the ctenidium and the pulmonary sac. The visceral ganglion is really a ganglionic mass formed by the fusion of two ganglia lying very close to anterior lobe of the digestive gland. The right pleuro-visceral connective lies below the oesophagus and is called as infra-intestinal visceral connective. The left pleuro-visceral connective lies above the oesophagus and is called as supra-intestinal visceral connective. The visceral ganglion gives off nerves to the renal organs, genital organ, pericardium, intestine and the digestive glands. 9. SENSE ORGANS The sense organs include two pairs of tentacles, osphradium, a pair of eyes, and a pair of statocyst. TENTACLES Two pair of tentacles is present in the head region. The first pair of tentacles is contractile and are known as labial palp while the second pair is longer, filamentous and retractile and

14 is the true tentacle, present behind the first one. Both the tentacles are tactile in nature. OSPHRADIUM There is only one osphradium that lies suspended from the roof of the mantle cavity on the left side. It is oval in shape with inner or right end bluntly rounded while the outer or left end pointed. It has a central axis to which two rows of thick, fleshy leaflets are attached. Leaflets are largest in the middle of the osphradium. In a transverse section, the osphradia is made up an outer layer of epithelial cells lined internally by a basement membrane. The epithelial cells are elongate and consist of three types of cells: 1. Sensory cells 2. Ciliated cells 3. Glandular cells STATOCYST A pair of small, pyriform, cream colored statocyst lie one on either side of the pleuro-pedal ganglionic mass. Each statocyst is a hollow capsule surrounded by thick, tough, leathery covering of connective tissue. The statocyst is ectodermal in origin, filled with a fluid containing calcareous particles known as statolith or statoconia. Statocyst is balancing organs and regulates the position in water and on land. It gets its nerve supply from the cerebral ganglion. EYES

15 A pair of short, black and stalked simple eyes is present one on either side of the head at the base of each true tentacle. In section, eye is in the form of a cup like invagination called the optic vesicle lined by photosensitive or pigmented retinal cells. The cavity of vesicle is filled with an oval hyaline structure called the lens. The anterior part of the optic vesicle in front of the lens is transparent and forms the inner cornea. The optic vesicle is surrounded by a firm sheath of connective tissue. The optic nerve enters the retina from the posterior end of the vesicle. 10. REPRODUCTIVE SYSTEM Sexes are separate and sexual dimorphism is distinct. The shell of the female is larger and more globular than the male. Males have a less swollen body whorl, and a more developed copulatory organ than the female. MALE REPRODUCTIVE SYSTEM A single cream colored testis lies in the upper two or three whorls in close contact with the digestive gland. The testis is a flat plate-like structure, more or less triangular in shape. Many fine ducts known as vasa efferentia originate from different parts of the testis. The vasa efferentia unite together to open into a common vas deferens. The vas deferens is divisible into three distinct parts: 1. The proximal thin walled tubular part 2. The middle slightly swollen seminal vesicle which stores the sperms 3. Terminal glandular part lying left of the rectum and opening into the mantle cavity close to the anus

16 The male copulatory organ is the penis which arises from the edge of the mantle in front of the anus and is enclosed in a penis sheath. The penis sheath is a simple outgrowth from the inner surface of the mantle. At the base of the penis sheath is a glandular thickening- the hypobranchial gland of unknown function. The spermatozoa of Pila are of two types: 1. Hair shaped or eupyrene sperm 2. Worm-like oligopyrene sperm Only the eupyrene sperms are functional and are capable of fertilizing the eggs. Also, the eupyrene sperm show a distinct head, a middle piece and a tail. The head and middle piece cannot be distinguished in oligopyrene sperm and also the tail bears four or five cilia. The oligopyrene sperms are non-motile and cannot fertilize the eggs. FEMALE REPRODUCTIVE SYSTEM A much-branched orange colored ovary is situated in the upper two or three whorls, attached to the inner surface of the digestive gland. The oviduct leads from the ovary and extends downwards along the digestive gland. It is differentiated into an upper tubular part and the lower glandular part that remains on the floor of the mantle cavity parallel to the rectum. The glandular part is distinguishable into an anterior yellow colored albumen gland that secretes albumen and a posterior part called as uterus. A bean-shaped receptaculum seminalis or the spermatheca is present at the junction of the tubular and glandular portion of the oviduct. The sperms

17 received from the male are temporary stored in the receptaculum seminalis after copulation. The terminal part of the uterus is differentiated into vagina. The vagina opens into the mantle cavity close to the anus by a female genital aperture situated on a small papilla. COPULATION Pila reproduces only during the rainy season. Copulation takes place on land, on the moist ground of the banks. During copulation, the male and female Pila come close together in such a way that the right nuchal lobe of one is opposed to the other. The copulation lasts for three to four hours at the end of which sperms are transferred from the seminal vesicle of the male to the receptaculum seminalis of the female where the sperms are temporarily stored. FERTILIZATION AND DEVELOPMENT Fertilization is internal and oviposition commences one or two days after copulation at a suitable place, such as moist earth near ponds. Fertilized eggs are laid in masses of SUMMARY Pila globosa is commonly known as pond snail or apple snail. It is typical representative of class Gastropods. It is one of the largest freshwater molluscs. It is commonly found in fresh water ponds, pools, tanks, lakes, marshes, rice fields and sometimes even in streams and rivers even in brackish waters of low salinity. The body consists of head, foot and a visceral mass. The digestive system comprises a tubular alimentary

18 canal, a pair of salivary glands and a large digestive gland. Pila is unique in having double mode of respiration, Aquatic by Ctenidium and Aerial by Pulmonary sac. The circulatory system is open and consists of heart, arteries, veins and the sinuses. Blood is colorless. Excretory system comprises of one or two kidneys and pericardial gland. Each kidney has two openings, one internally communicating to the pericardium and the other to the exterior called the nephridiopore. The nervous system consists of a series of paired ganglia with commissures and connectives. The sense organs include two pairs of tentacles, osphradium, a pair of eyes, and a pair of statocyst. Sexes are separate but sexual dimorphism is almost absent.

A New Terrestrial Prosobranch Family (Tutuilanidae) I from Samoa, with Descriptions of a New Genus and a New Species*

OCCASIONAL PAPERS OF BERNICE P. BISHOP MUSEUM HONOLULU, HAWAII Volume XX June 13, 1952 Number 18 A New Terrestrial Prosobranch Family (Tutuilanidae) I from Samoa, with Descriptions of a New Genus and a