Chapter 16 MORPHOLOGY OF ANIMALS

Transcription

1 Chapter 16 MORPHOLOGY OF ANIMALS Earthworm (Pheretima posthuma) They are nocturnal, hermaphrodite and burrowing (fussorial), animal which lives in wet soil with humus and called as nature s ploughman. Body is long (15 20 cm) cylindrical, segmented ( 120) and regionated in pre-clitellar, clitellar and post-clitellar parts. Clitellum covers fourteenth to sixteenth segments. Color is brown due to presence of porphyrin pigment in the skin which protects the body from injurious effect of sun light. Ventral surface is marked by genital apertures while the dorsal surface is marked by deep colored blood vessel. Body of earthworm : (a) dorsal view (b) ventral view (c) lateral view showing mouth opening Except first, last and clitellar segments each segment bears a single row of tiny curved setae, about in each. First segment, the peristomium, has mouth opening, below dorsal hood-like prostomium. In the inter-segmental groove of all segments form 12 last but one there is single mid-dorsal pore in each groove. Coelomic fluid oozing through it keeps the body surface moist. Four pairs of spermaithecal pores lie in the inter-segmental grove on ventro-lateral sides of 5/6, 6/7, 7/8, 8/9 segments. On the mid-ventral part of 14th segment is a female genital pore; on the ventrolateral sides 17th and 19th segments each there is a pair of male genital papillae; on 18th segment there is one pair of male genital pore. In each segment behind 6th there are about nephridial pores of integumentary nephridia. Coelomic fluid also serves as hydraulic skeleton.

2 Alimentary canal is straight tube-like, pharynx, gizzard and intestine as main parts. Pharynx is a large, bulbous and glandular part spanning 3rd and 4th segments. Its dorsal wall is thick and provided with glands to secrets saliva and enzymes. The lumen remains divided into dorsal salivary chamber and ventral food chambers. Gizzard is a muscular, bulb-like structure for grinding the food, internally lined with hard cuticle layer and wall with thick circular muscular layer. Intestine extends from 15th to the last segment is divided as pre typhloslar, typhlosolar and post typhlosolar. Typhlosole is the absorptive structure present in segments from 25th to th. It feeds upon the humus (organic matter) in the soil. Alimentary canal of Earthworm Mouth Buccal chamber Brain Pharynx Oesophagus Obligue septum Gizzard Stomach Pretyphlosolar intestine Intestinal cecae Typhlosolar intestine Dorsal blood vessel Lymph glnds Typhlosole T.S. Gizzard T.S. Intestine showing typhlosole Circulatory system is closed type with four pairs of pulsating heart in 7th, 9th, 12th and 13th segments each; first two pairs are lateral heart and the rest two pairs are lateral oesophageal heart. The main blood vessels are dorsal vessel, ventral vessel, lateral oesophageal and sub-neural vessels. Dorsal vessel is thick walled, pulsatile and with valves, blood flows from posterior to anterior direction in it, before 13th segment it acts as distributing channel and after 13th as collecting

3 channel. Ventral vessel acts as distributing channel and blood in its flows from anterior to posterior direction. Lateral oesophageal vessel run along the lateral side of gut upto 13th segment and after this both vessels unite to form a single sub neural vessel which runs along rest of body length below nerve cord. The shortest vessel in supra-esophageal vessel from 9th to 13 segment just dorsal to stomach. Blood vascular system in Earthworm On the dorsal side of pharynx there are blood glands one each in 4, 5 & 6 segments, these form blood corpuscles and haemoglobin. Chloragogen cells in the coelom earlier said to have excretory functions are actually analogous to vertebrate liver. Excretory organ is nephridia of three types: (i) Integumentary nephridia (exonephric type), (ii) Pharyngeal nephridia and (iii) Septal nephridia (both are entero-nephric type). Integumentary nephridia are present in each segment behind 6th segment along the inner surface of body wall. Their number varies from 200 to 250 in each segment, but in clitellar region the number is 2000 to 2500 per segment, it is therefore, referred to as forest of nephridia. Mouth Buccal Chamber Pharynx Oesophagus Gizzard Stomach Ducts Pharyngeal Nephridia Blood Glands Tufts of Pharyngeal Nephridia Integuments Nephridia Twisted Lobe Apical Limb Distal Limb Straight Lobe Ciliated Canal Nephrostome Proximal Limb Clitellum Intestine Septal Nephridia Neck Terminal Excretory Duct Nephridia in Earthworm Septal nephridia in Earthworm

4 Septal nephridia are the typical and largest, about 80 to 100 per segment along the septum in each segment behind 15th to the last segment. Pharyngeal nephridia are present as paired bunches on dorso-lateral sides of pharynx in 4.5 and 6th segments. Resemble septal nephridia in size and integumentary nephridia in structure and open through 3 pairs of common ducts into pharynx. Excretion is both ammonotelic and ureotelic. Nervous system represents typical advance structure distinct in central and peripheral components. CNS (Central Nervous System) consists of a nerve ring (brain) with supra-pharyngeal and subpharyngeal ganglia connected by circumpharyngeal ring. A slender ventral nerve cord with segmental ganglionic swelling runs along entire length of body. Photoreceptors occur in deep layer epidermis of skin (dorsal) in the anterior and posterior ends of the body. Reproductive system is well developed having both male and female systems in the same individual i.e. Hermaphrodite (monoecious or bisexual). Male organ consist two pairs of testes each in 10th and 11th segments; two pairs of testis sac, seminal vesicles and vasa deferentia opening on male genital pore; a pair of prostate gland. Female reproductive organs consist of a pair of ovary located in 13th segment. Behind each ovary there is oviducal funnels leading to short oviducts. Both oviducts meet together and open by a common female genital pore in 14th segment. 4 pairs of spermatheca are present, one pair in each segments from 6th to 9th. These store sperm from other partner. Protandrous, only cross-fertilization occurs externally within the cocoon. Development also takes place in cocoon. In 2-3 weeks the embryo becomes adult, normally only one young one develops in one cocoon Reproductive system of Earthworm

5 Cockroach (Periplaneta americana) As a model insect the features described here is mainly about the American cockroach, Periplaneta americana (common cockroach). The other types are Blatta orientalis (the Indian cockroach) and Blattela germanica (the German cockroach) Periplaneta is the largest, also known as Bombay canary or the ship cockroach. It is nocturnal, cursorial and omnivorous in habit and cosmopolitan in distribution; exhibits sexual dimorphism; lives mainly in dark, damp and warm places like kitchen, stores, sewers, etc. Measures about cm in length and 1.5 to 2 cm in width. Body is divisible into head, thorax and abdomen. Embryonically head is made of 6 segments; thorax of 3 and abdomen of 11 segments. In adult head is single piece structure, thorax of 3 segments and abdomen of 10 segments. Body covering is in the form of cuticle pieces (sclerites) jointed by arthrodial membrane. Dorsal sclerites are called terga (sing. tergum); the ventral are sterna (sing. sternum) and the lateral are pleura (sing. pleuron). Inner sclerites are tantorium in head as base of brain and apodemes in other parts as fulcrum. External features of Periplaneta A. male in dorsal view, B. female in ventral view Distinguishing features between male and female cockroach Male Female Comparatively smaller in size Larger in size Abdomen slender Abdomen wider posteriorly One pair anal style present Anal style absent 7th sternite normal, doesn t cover 7th sternite large, boat shaped, covers the 8th & 9th sternites the 8th & 9th sternites completely and forms genital chamber (gynatrium) Around the genital pore there are A few gynovalvular plates around 3 phallomeres which help the genital pore hold the in copulation spermatophore and help in deposition of ootheca into soil.

6 Head Hypognathous, covered with sclerites like epicranium or vertex; frons, clypeus, as pair of gena. One pair of large compound eye is present on each dorsolateral side of head followed by a pair of fenestra (ocellar spot). A pair of long filiform (setacious) antenna representing the appendages of 2nd segment of head is present as sensory organ. Antenna is made of 3 main segments; the proximal scape, the middle pedicel and terminal flagellum. Head of cockroach Parts of head region Thorax Divided as prothorax, mesothorax, and metathorax; dorsal sclerites are nota (sing. notum) to which wings are attached. Prothorax is covered with largest pronotum and without any wing. Mesothorax consists of a pair of wings and 2nd pair of legs but 1st pair of leg. Metathorax consists of 2nd pair of wings and 3rd pair of legs. Wings Ectodermal, cuticular structure with nervures (veins) and tracheal tubes: there is definite pattern of venation. Haemocoelomic fluid flows in veins.

7 Fore wings (1 st pair or mesothoracic wing) is thick, leathery also called as elytra or tegmina, has no flying function but covers the body and hind wing. Hind wings (2 nd pair or metathoracic wing) is membranous and with true flying function. Leg Uniramous, thoracic appendages remain attached to ventro-lateral sclerite through coxa; each consists of 5 segments; coxa trochanter femur tibia tarsus. Tarsus is subdivided into 5 tarsomeres. The terminal tarsomere is called pretarsus with a pair of claws and a median pad-like arolium (or pulvillus). At the joints of tarsomeres are attached leaf-like plantulae. Arolium and plantulae help in moving along the plane, smooth surface like glass and along the ceiling. Femur Trochanter Tibia Coxa Tarsus Leg of cockroach Abdomen There are 10 tergites, 8th and 9th remain covered by 7th; between 5th and 6th tergites there is a pair of stink gland (or repugnatorial gland) in male which secretes the repelling substance for other male. 10th tergite is bilobed and in both male and female bears a pair of 15-segmented anal circi having sensory hairs for receiving sound vibrations and heat. Sternites are only 9 in male and 7 in female. The 7th sternite in female is large boat shaped which covers 8th & 9th sternites completely and form genital chamber, gynatrium; its posterior part is oothecal chamber. The 9th sternite in male bears a pair of anal style. Mouth Parts These are the modified head appendages adapted for collection and manipulation of food. It is biting and chewing type also called a mandibulate type or orthopterous type. Labrum is a single piece, plate like, attached to clypeus; makes roof of the preoral cavity (POC) Mandible: one pair, hard, movable structure with molar and incisor processes to cut and chew the food, makes upper half (dorso-lateral part) of the wall of POC. Maxilla: one pair as lower half of the wall of POC, is a typical biramous structure; basopodite divided as proximal cardo and distal stipes exopodite is 5 segmented maxillary palp; endopodite is branched with outer galea and inner lacinia; meant for collection, manipulation and tasting of food. Hypopharynx is single torque-like cuticular part in the middle of POC, not as appendages; possess salivary channel. Mouth parts of cockroach

8 Labium: One pair both fused together to make floor of POC; also called as 2nd pair of maxilla as similar in structure and function; basopodite divided as submentum, mentum and prementum; exopodites is 3- segmented labial palp; endopodite bifurcated as outer paraglossa and inner glossa. Digestive System Digestive system consists of alimentary canal and salivary glands, gut is divided initially as foregut (stomodaeum), midgut (mesenteron) and hindgut (proctodaeum) on the basis of origin. Foregut is ectodermal having cuticular lining and divided as mouth, pharynx, esophagus, crop, and gizzard. Crop is the largest part analogous to vertebrate stomach where food is stored. There is no digestive gland here but digestion takes place with the help of enzymes coming from the midgut or hepatic caeca. Gizzard (proventriculus) is the small, bulbous and highly muscular part with six hard cuticular teeth inside for cutting and mastication of food. Stomodeal valves at the end of foregut extends into the upper part of midgut, it Digestive system of cockroach secretes peritrophic membrane. The beginning of midgut is externally marked by the presence of hepatic caeca as blind tube. Midgut (ventriculus or stomach) the long coiled tube of uniform diameter is the actual alimentary canal (endodermal) which secretes digestive enzymes and absorb digested food. Inside the lumen food remains encircled by peritrophic membrane which is a porous cuticular membrane to protect the midgut mucosa from abrasion but, it puts no barrier against digestive enzyme and digested food. Hindgut is divided as ileum, colon and rectum all adapted for absorption of water from the outgoing faecal part. Externally its demarcation from midgut is made by presence of hair-like Malpighian tubules (90 150). Cockroach is omnivorous and hence all possible kind of digestive enzymes except cellulose is found, e.g. amylase, lipase, protease, nuclease etc. L.S. through foregut and midgut Respiratory System Includes a system of highly branched cuticular tubes called trachea which is able to carry gases directly to the tissue where it branches as tracheoles which is without cuticular rings. There are six main longitudinal trunks. Trachea communicate to outside through 10 pairs of spiracles (stigmata) guarded with valves. 2 pairs spiracles are present on the dorsolateral surface in the intersegmental groove (1/2 &

9 2/3) of thoracic segments and 8 pairs in the abdominal segments which mostly remain closed but open only during inspiration. Breathing is added by contraction of tergo-sternal muscle. While at rest tracheolar capillaries contains fluid with raised level but when in activity the fluid subsides because of high pressure of air in it. Circulatory System It is open (or lacunar) type, blood flows in haemocoelomic sinuses. Heart is 13 chambered, long tubular, pulsatile structure present in the dorsal pericardial sinus just below tergum. Each chamber belongs to each thoracic and abdominal segments and opens by a pair of ostia. Wider end lies posteriorly and narrow end anteriorly. Blood flows from posterior to anterior end. The narrow end of first chamber enters into head as dorsal aorta. A pair of fan-shaped alary muscle is associated with each chamber connected to tergum and the dorsal diaphragm. Contraction of these muscles aid in circulating blood. Contraction of tergo-sternal muscle also adds to circulation of blood. Haemocoel is divided by dorsal and ventral diaphragms into 3 Heart of cockroach sinuses: dorsal pericardial sinus, middle peri-visceral sinus and ventral peri-neural sinus. Being perforated the diaphragms allow free flow of blood from one to another sinuses. Blood (haemolymph) is colourless without respiratory pigment. Plasma has 70% water and rest protein, amino acids, lipids and carbohydrates. A special type of disaccharide sugar, trehalose is present as reserve food. The cellular component consists of mainly leucocytes.

10 Fat bodies are the special mass of cells found in the haemocoel mainly for the storage of fat for other functions as per following cell types it is analogous to vertebrate liver. These cells are also found free in haemolymph. Trophocytes store food (glycogen, protein, amino acids, lipids etc.) Mycetocytes contain symbiotic bacteria which breaks uric acid to synthesize vitamin, amino acid. Oenocytes contain and secrete lipid (wax) and metabolic wastes. Urate cells absorb uric acid from haemolymph and store it. Nephrocytes remove uric acid from haemolymph, in pericardial sinus around heart. Excretory system Malpighian tubules, the main excretory organ are analogous to enteronephric nephridia of earthworm. Ectodermal in origin, the wall of tubule is made of cuboidal epithelium ; its distal end absorbs potassium urate from haemolymph and secretes it into the lumen. Here combining with H 2 O and CO 2 it forms uric acid and KHCO 3. Uric acid is drained into hindgut while KHCO 3 goes back into haemolymph. Mushroom gland, cuticle, urate cells, nephrocytes are additional means of excretion. Compound Eye One pair, ectodermal, kidney shaped structure present on dorso-lateral side of head; Each consists of 2000 small, convex, hexagonal facets representing the unit eye called ommatidium. All ommatidia are parallel, longitudinal and radially arranged, separated by pigmented sheath layer. Divided as upper dioptrical and lower sensory (or receptor) part. Outermost cuticular layer, cornea, acts as lens, it is secreted by the two underlying corneagen cells. The dioptrical part has 4 cone cells (vitrellae) surrounding a crystalline cone secreted by them. The receptor part 7 retinal cells (retinulae) surrounding a solid rhabdome. The retinal cells have nerve connection at the base. Since in cockroach the pigment in the sheath layer remains in permanently spread state so only apposition image or mosaic vision is possible.

11 In other insects in day light it is apposition image is formed which in night (dim light) it is the superposition image. Nervous System It is of typical annelidan type, ganglia of head segments constitute brain; 3 thoracic ganglion and 6 abdominal ganglia are connected by ventral double nerve cord. Sympathetic or Visceral nervous system consists of stomatogastric, spiracular and caudal nervous system. Stomatogastric component consists of five ganglia and their interconnecting nerves. These ganglia are: (i) frontal (ii) hypo-cerebral, (iii) visceral and (iv) pro-ventricular. Endocrine System It consists of various glands viz: (i) Neurosecretory cells in brain; (ii) Corpora allata (iii) Corpora cardiaca and (iv) Prothoracic gland Corpora allata secretes neotenin or juvenile hormone which controls growth in the metamorphosis. Corpora cardiaca stores the neurosecretory substances from brain and controls rest all physiological processes like circulation, heart, reproduction, metabolism, etc. Prothoracic gland secretes ecdysone or moulting hormone Certain pheromones (also wrongly called as ectohormones) controls the behaviour and sexual act. Reproductive System Reproduction and behavior show high degree of organisation like higher vertebrates. Male reproduction system consists of a pair of testis lying in 3rd to 8th segments dorso laterally, a pair of vas differences and a common ejaculatory duct opening on 3rd phallomere by gentital pore. Mushroom (or utricular) gland has short central and long peripheral tubules respectively called as utriculi breviores and utriculi majores, secreting the medium of sperm and innermost layer of spermatophore. The middle layer of spermatophore is secreted by ejaculatory duct.

12 Phallic (or conglobate) gland is a long club-shaped gland opening upon the left phallomere secretes outer most layer of the spermatophore. Female reproductive system Male reproductive system Female reproductive system consists of a pair of ovary, each with 8 ovarioles and a short oviduct which meet to form a common oviduct or vagina which opens by female genital pore in gynatrium. In cone pair colleterial gland left is larger and highly branched than right one. Left gland secretes milky protein; the right gland secretes dihydroxyphenol. Both combine to form scleroprotein, as the covering of ootheca. Ootheca is the egg case with 16 eggs arranged in 2 tiers like cigarette in its case. Each female produces 15 to 40 ootheca during its life span of about 2 years. Spermatheca (recepticula seminalis) is a pair of small sac like structures. Left is larger than right one. It stores sperm coming from the spermatophore of male deposited in the genital chamber. Copulation March to September is active breeding season. Male gets attracted by the pheromones from female. Arousal of male is marked with fluttering of wing and dancing around the female. Male slips beneath the female from front side and brings its hind end beneath that of female, seems attached back-to-back. With the help of titilator, pseudopenis on the left phallomeres and serrate lobe on right phallomere introduces spermatophore within the genital chamber of female. Eggs coming in batches of 16 get feritilized and encased into ootheca. After about 20 hrs sperm is transferred into spermatheca and the empty spermatophore is dropped. Female deposits ootheca in a dark, dry and safe place within the soil. Life Cycle The development in ootheca is completed within 5 to 13 weeks depending upon the environmental condition mainly temperature. It is indirect involves hermimetabolous type of metamorphosis. 16 nymphs hatch out from ootheca. Nymphs are similar to adult except the smaller size, absence of wings and reproductive organs. After 10 to 12 moultings the nymph changes into adult which may take 6 months to 2 years time.

13 Frog (Rana tigrina) Frog is a cosmopolitan, poikilothermic, insectivorous animal most favour for the study of Zoology. Body in divided into two parts; head and trunk. The absence of neck is the adaptation to aquatic life. Head is triangular at anterior end called snout with mouth bounded by upper and lower jaws. Paired nostrils are positioned near the snout. Elevated and round eye is situated behind each external nostril slightly posterior to eye, a depression remains covered by tough and grey membrane called as tympanum or tympanic membrane. Eyes of frog possess following three types of eyelids: Upper eyelid (thick and immovable) Lower eyelid (small and movable) Third eyelid or nictitating membrane (transparent and very thin) Nictitating membrane movement is controlled by a set two antagonstric muscles; retractor bulbi (for unfolding of nictitating membrane) and levator bulbic (for folding of nititating membrane). Eyelids lack eye lashes Trunk is the main part of body with two pairs of legs: Fore limb is short and with four is short and with four digits. Thumb is absent. In made thick pad develops in the palm to help in copulation. Hind limb is stronger and longer divisible into thigh, shank and food with ankle, sole and 5 toes. Toes are connected by a web, which help in swimming. The posterior end of trunk, between the hind limbs has an aperture called cloacal aperture for the release of faeces, urine and genital products (sperms and ova) to outside the body. Differentiating features of male and female frog Male Female Larger in size Smaller than male Large and more muscular present A pai of vocal sac Present below power jaw Vocal sac absent. Copulatory pad formed No capulatory pad formed fore limb as swelling below first finger during breeding season

14 The skin is shining and moist. Skin on the ventral surface of body is faint yellow and dorsal surface is greenish with black spots. The skin of dorsal surface contains ridges due to the thickening of skin are called dorso-lateral dermal plicae. Skin in scaleless mucous and poison glands are present in the dermal region. The secretion of mucous keeps skin moist, glistening and sticky whereas poison glands secrete a mild poison to protect from enemies. Skull is dicondylic, platybasic (an inter-orbital septum is absent and cranium extends beyond orbits). Upper jaw consists of premaxilla with 4 5 teeth, maxilla with numerous teeth and quadratojungls jugals (comma-shaped), are positioned at outer margin of upper jaw Lower jaw comprises mento-meckelian (at the tip), dentary (outer surface) and angulosplenial pairs of bones with articular bone at the end. Quadrate is found at the angle of upper jaw. It attachs to lower articular bone of jaw and called as autostylic suspensorium. Teeth are altogether absent in the lower jaw but present in upper jaw. These are maxillary and vomerine teeth. Tooth is of acrodont, homodont and polyphyodont type. In toad there is not tooth. Sternum articulates directly with the pectoral girdle. Sternum is made up of episternum (anterior flattened circular catrilage), omosternum (inverted Y shaped cartilage bone), mesosternum (rod-like cartilage bone) and xiphisternum (terminal circular cartilage) Contraction of sternohyal muscles (lowering of oral cavity) and petrohyal muscles (raising of the floor of buccal cavity) is responsible for breathing External features of Rana tigrina Diagrammatic representation of internal organs of frog showing complete digestive system

15 Glottis is controlled by muscles attached to arytenoid cartilages Brain and spinal cord are covered by duramater and piamater, no arachnoid layer. Mid brain has a pair of large optic lobes (corpora bigemina) with optocoel. Pineal body is well developed on epithalamus. Cerebellum small transverse cylindrial structure with cavity. There is no pons varoli. Bifid and retractile tongue, no chemical process of digestion in buccal cavity (buccal epithelum lacks digestive glands) Heart 3 chambered (2 auricle + 1 ventricle) contains coronary sulcus (separating aruicles from ventricle), truncus arteriosus (with pylangium and synangium) on ventral side and sinus venosus on dorsal side. Ventricle is undivided. Only mixed blood flows in the body of frog. Kidneys are pronephros (in tadpole) and opisthonephros (in adult) which is actually mesonephros Four pairs of renal arteries and one pair of renal portal veins supply blood to kidneys. Blood from kidneys is collected by four pairs of renal veins. Male reproductive system Female reproductive system Fertilisation is external. During fertilisation several spermatozoa encircle the ovum and one of them succeeds in entering it. Mass of sperms released by male during copulation is called milt. Mass of ova released by female during copulation is called frog spawn. Zygote or egg is spherical and measures about 1.6 mm in diameter. The upper half of the zygote is black (pigmented) and contains nucleus and cytoplasm. The lower part of the zygote is white or light yellow in colour and contains the reserve foods in the form of yolk. It is a unisexual animal. It exhibits sexual dimorphism during breeding season. Zygote floats in water and incubated by sun light. Each egg undergoes several cleavages, passed through blastula and gastrula stages and after organogenesis, a larva is developed inside the egg. In few days the larva hatched out. Larva of frog is called as tadpole larva. Newly hatched tadpole larva measures 0.5 mm in length. On the ventral side of head a horse shoe shaped sucker is present, with which it is attached to leaves of aquatic plants.

16 In external gill stage mouth is formed which is provided with a pair of horney jaws. Now it starts feeding on the leaves of aquatic plants. (herbivorous). The larva grow in size. Tail becomes elongated, developes a caudal fin. Then larva swims freely in water with the help of tail with its caudal fin. Three pairs of external gills are functional and respiratory in function. The intestine becomes very long and coiled like a spring of watch. In internal gills stage larva grow in size four pairs of gill slits are formed in the region of pharynx. Operculum opens out through a branchial aperture situated on the left side. The tadpole resembles a fish in all aspects. In the mean time, fore limbs and hind limbs appeared as buds. Fore limb buds are covered by opercula and hence they are not visible in the early stage. Larva respires with gills for some time and with lungs for some time. The changes that takes place during transformation of larva into adult are collectively called metamorphosis. During metamorphosis, tadpole undergoes the following changes: Disappearance tail of starts The horny jaws are replaced by bony jaws Tongue becomes long and eyes becomes protruberant The alimentary canal shortens to suit for carnivorous food habit. Gills disappear, skin develops respiratory function for aquatic life. Lungs become functional for terrestrial life Liver and pancreas become fully developed and functional Pronephric kidney changes into mesonephric type Excretory waste changes into urea, instead of NH 3. Fore limbs come out side the operculum Hind limbs grow longer with web in between the digits Cartilagenous skeleton changes into bony. Lateral line organs disappear Changes take place in blood vascular system as per the changes of respiratory stem. Heart becomes 3 chambered, arteries formed, branchial arteries disappear.