Life and Diversity of Animals – Chordates (Protochordata to Amphibia) BZO3T05

Syllabus topics (RTMNU)

• 1.1 Origin and ancestry of Chordata
• 1.2 General character and classification of Chordata upto classes, affinities of protochordates with Hemichordata and Agnatha.
• 1.3 Herdmania: digestive system, ascidian tadpole and retrogressive metamorphosis.
• 1.4 Amphioxus: digestive system, sense organs (Kolliker's pit and infundibular organ) and protonephridia.

1.1 Origin and ancestry of Chordata

Chordata are animals that have a notochord, a dorsal hollow nerve cord and pharyngeal gill slits at some stage of life. Early forms were soft-bodied and left few fossils, so many theories exist.

Main theories of origin

• Annelid theory (Dohrn, Semper): chordates came from annelid-like segmented worms. If an annelid is turned upside down, its ventral nerve cord becomes dorsal and its dorsal blood flow becomes ventral, like a chordate. Objection: annelid nerve cord is solid and double, mouth would come on the dorsal side, and there is no notochord.
• Arachnid theory (Gaskell, Patten): chordates came from arachnid-like arthropods such as Limulus or extinct ostracoderm-like eurypterids. Objection: arthropod gut cannot become a neural tube.
• Echinoderm theory (Garstang, Berrill, de Beer): most accepted. Based on embryology. Echinoderms and chordates are both deuterostomes: radial and indeterminate cleavage, blastopore forms anus, enterocoelous coelom, mesodermal endoskeleton. The Auricularia larva of holothurians and the Tornaria larva of Balanoglossus are almost alike.

Garstang's larval (paedomorphosis) theory

1. An auricularia-like larva of an echinoderm ancestor had ciliated bands.
2. The ciliated bands and the nerve tracts below them moved up and fused on the dorsal side to form a dorsal hollow nerve tube.
3. Muscles and a stiff rod (notochord) developed in the tail for swimming, giving a tadpole-like larva like the ascidian tadpole.
4. By paedomorphosis/neoteny (larva becoming sexually mature), this free-swimming larva never settled and gave rise to free-living chordates.

Conclusion: chordates and echinoderms share a common deuterostome ancestor.

1.2 General characters and classification of Chordata; affinities of protochordates

Three fundamental characters

• Notochord: a flexible, rod-like supporting structure, dorsal to gut, made of vacuolated cells in a sheath. Replaced by vertebral column in vertebrates.
• Dorsal tubular nerve cord: single, hollow, dorsal to notochord (in non-chordates it is solid, ventral, often double).
• Pharyngeal gill slits: paired openings in the pharynx wall; used for respiration or filter feeding.

Other characters

• Bilaterally symmetrical, triploblastic, enterocoelous coelomates, deuterostomes.
• Post-anal tail present at least in embryo.
• Heart ventral; closed circulation; hepatic portal system present.
• Endostyle or thyroid gland present.

Classification up to classes (commonly taught Indian textbook scheme)

Note: older books keep Hemichordata inside Chordata; modern books make it a separate phylum because its stomochord is not a true notochord.

Affinities of protochordates with Hemichordata

• Pharyngeal gill slits in both.
• Dorsal hollow nerve cord in collar region of Balanoglossus.
• Deuterostome, enterocoelous development.
• Differences: stomochord is not a true notochord; no post-anal tail; nerve cord mainly solid.

Affinities of protochordates with Agnatha

• Endostyle of Amphioxus and ascidians is the same as the endostyle of the ammocoete larva, which becomes the thyroid.
• Filter feeding with velum and mucus in ammocoete and Amphioxus.
• Persistent notochord, myotomes, no jaws, no paired fins.
• Agnatha are more advanced: cranium, brain, paired eyes and ears, vertebral elements, kidneys.

1.3 Herdmania: digestive system, ascidian tadpole and retrogressive metamorphosis

Herdmania pallida is a sessile, marine, solitary ascidian (sea squirt) found on the Indian coast. Body covered by a test of tunicin.

Digestive system

1. Branchial siphon → mouth → branchial sac (pharynx), a large perforated basket with many stigmata (gill slits).
2. Endostyle (ventral groove) secretes mucus; dorsal lamina on dorsal side; peripharyngeal bands join them.
3. Oesophagus → stomach → intestine (U-shaped loop) → rectum → anus opens into the atrium.
4. Pyloric gland (digestive gland) on the intestine.

Feeding (ciliary/filter): cilia draw water in; food particles are trapped in mucus sheets from the endostyle; the mucus string rolls up along the dorsal lamina into the oesophagus. Water passes through stigmata into the atrium and out through the atrial siphon.

Ascidian tadpole larva

• Free-swimming, non-feeding, lives a few hours to 1–2 days.
• Has all chordate characters: notochord in the tail, dorsal hollow nerve cord with a cerebral vesicle, pharynx with gill slits, post-anal tail with muscles.
• Sense organs: ocellus (light) and otolith (balance) in the sensory vesicle.
• Three adhesive papillae at the anterior end for attachment.

Retrogressive metamorphosis

Definition: a change in which an active, advanced larva becomes a simpler, degenerate, sessile adult.

1. Larva attaches by adhesive papillae.
2. Tail with notochord, nerve cord and muscles is absorbed (eaten up by phagocytes).
3. Nerve cord reduces to a single neural ganglion; sense organs disappear.
4. Rapid growth between mouth and attachment point rotates the body by about 180°, so the mouth (branchial siphon) comes to the top.
5. Pharynx enlarges and gill slits increase; atrium and test develop.

Significance: the larva shows that Herdmania is a true chordate; the adult has lost most chordate features. Called "retrogressive" because it goes from advanced to degenerate.

1.4 Amphioxus: digestive system, sense organs and protonephridia

Amphioxus (Branchiostoma), the lancelet, is a small marine cephalochordate that burrows in sand. Notochord runs from snout to tail tip.

Digestive system

• Oral hood with buccal cirri (sieve) → vestibule with wheel organ (ciliated tracts) and Hatschek's pit → velum with velar tentacles → mouth.
• Pharynx with about 180 pairs of gill slits; endostyle (ventral), epipharyngeal groove (dorsal), peripharyngeal bands.
• Oesophagus → midgut diverticulum (hepatic caecum) → ileocolic ring → intestine → anus on left side.
• Filter feeding as in Herdmania; water goes out through the atriopore. Hepatic caecum digests food intracellularly too.

Sense organs

• Kolliker's pit (olfactory pit): a ciliated depression on the left side of the snout above the cerebral vesicle; thought to be chemoreceptive (smell). It is the remains of the neuropore.
• Infundibular organ: a group of ciliated cells on the floor of the cerebral vesicle; thought to sense pressure of the cerebrospinal fluid. Compared with the infundibulum of vertebrate brain.
• Others: frontal eye spot; Hesse's ocelli along the nerve cord; tactile cells on cirri.

Protonephridia (excretion)

• About 90–100 pairs of nephridia above the pharynx, one for each pair of primary gill bars.
• Each has many solenocytes (flame-cell-like cells with a long flagellum in a tube) projecting into the coelom.
• Waste from coelomic fluid is driven by flagella into the nephridial canal, which opens into the atrium by a nephridiopore.
• Significance: solenocytes resemble protonephridia of annelids/flatworms (invertebrate type), unlike vertebrate kidneys.

Likely exam questions

1. (Long, 8 marks) Describe the theories of origin of Chordata. Explain Garstang's echinoderm (larval) theory.
2. (Long, 8 marks) Describe the ascidian tadpole larva of Herdmania and its retrogressive metamorphosis with diagrams.
3. (Short, 4 marks) Write the three fundamental characters of Chordata.
4. (Short, 4 marks) Write a note on Kolliker's pit and infundibular organ of Amphioxus.
5. (Short, 4 marks) Describe the protonephridia (solenocytes) of Amphioxus.

Syllabus topics (RTMNU)

• 2.1 Vertebrata: General characters, affinities of vertebrata with protochordata.
• 2.2 Petromyzon: Digestive system, Respiratory system, Reproductive system.
• 2.3 Ammocoete larva, General characters of Myxine.
• 2.4 Ostracoderm: General characters with examples, reason of extinction.

2.1 Vertebrata: general characters; affinities with Protochordata

Vertebrata (Craniata) are chordates in which the notochord is surrounded or replaced by a vertebral column and the brain is enclosed in a cranium.

General characters

• Body divided into head, trunk and tail (neck in many tetrapods).
• Endoskeleton of cartilage or bone: skull, vertebral column, girdles and limbs.
• Well-developed brain (fore-, mid-, hind-brain) with 10 or 12 pairs of cranial nerves.
• Paired sense organs: eyes, ears (also balance), nose.
• Ventral, muscular, chambered heart (2, 3 or 4 chambers); red blood cells with haemoglobin; closed circulation with hepatic portal system.
• Respiration by gills (aquatic) or lungs (terrestrial).
• Excretion by kidneys (pronephros, mesonephros or metanephros).
• Endocrine glands present: pituitary, thyroid, adrenal etc.
• Usually two pairs of appendages (fins or limbs); sexes separate.

Affinities with Protochordata (similarities)

• Notochord, dorsal hollow nerve cord and pharyngeal gill slits at least in the embryo.
• Post-anal tail and myotomes.
• Endostyle of protochordates = thyroid of vertebrates (proved by ammocoete larva).
• Hepatic caecum of Amphioxus is comparable to the vertebrate liver.
• Ventral heart / ventral blood flow forward, dorsal blood flow backward.

Differences (vertebrates are advanced)

2.2 Petromyzon: digestive, respiratory and reproductive systems

Petromyzon marinus (sea lamprey) is a jawless cyclostome. It is an ectoparasite on fishes: it attaches by a suctorial buccal funnel, rasps the flesh and sucks blood. It is anadromous (lives in sea, breeds in rivers).

Digestive system

• Buccal funnel (sucker) lined with horny epidermal teeth; edge has papillae.
• Tongue (piston) with horny teeth works like a rasp.
• Mouth → buccal cavity → pharynx divides into a dorsal oesophagus and a ventral blind respiratory tube. A velum guards the opening of the respiratory tube.
• Oesophagus → straight intestine with a fold, the typhlosole (increases surface) → cloaca. No true stomach.
• Paired buccal (salivary) glands secrete lamphredin, an anticoagulant that keeps the host's blood flowing.
• Liver present; gall bladder absent in adult.

Respiratory system

• Seven pairs of gill pouches (sacs) open into the respiratory tube internally and outside by seven pairs of external gill slits.
• Gill pouches are lined with gill lamellae rich in capillaries; supported by a branchial basket of cartilage.
• Tidal respiration: when the lamprey is attached and the mouth is closed, water is pumped in and out through the external gill slits by contraction and expansion of the branchial basket.

Reproductive system

• Sexes separate. A single, large, median gonad (testis or ovary) without a duct.
• Gametes are shed into the coelom → pass through two genital pores → urinogenital sinus → urinogenital papilla → outside.
• Breeding in fresh water; male and female build a nest of stones; external fertilisation. Adults die after spawning.
• Development indirect through the ammocoete larva.

2.3 Ammocoete larva; general characters of Myxine

Ammocoete larva

• Larva of Petromyzon; lives buried in mud of streams for 3–7 years; worm-like, blind (eyes under skin).
• Oral hood instead of sucker; no teeth; velum pumps water.
• Filter feeder like Amphioxus: food trapped in mucus.
• Endostyle (subpharyngeal gland) on floor of pharynx produces mucus and binds iodine; at metamorphosis it becomes the thyroid gland.
• Pharynx not divided; 7 pairs of gill slits.
• Undergoes metamorphosis into the adult lamprey.

Significance: it is a link between protochordates (Amphioxus) and vertebrates. Its resemblance to Amphioxus (filter feeding, endostyle, velum) shows vertebrates came from protochordate-like ancestors.

General characters of Myxine (hagfish)

• Marine, burrowing; a scavenger / internal parasite of dead and dying fish.
• Mouth terminal, surrounded by 4 pairs of tentacles; no buccal funnel.
• Single nostril opens into the pharynx (naso-pharyngeal duct) – used for breathing.
• 6 pairs of gill pouches, which open outside by a single common gill opening on each side.
• Eyes degenerate, under the skin.
• Large slime glands along the body – produce much mucus for defence.
• Development direct (no larva); large yolky eggs; single gonad; young are hermaphrodite, adults work as one sex.

2.4 Ostracoderms: general characters, examples, reason of extinction

Ostracoderms ("shell-skinned") are the earliest known vertebrates; extinct jawless fishes of the Ordovician to Devonian period.

General characters

• Small (mostly 10–50 cm), fish-like, bottom-dwelling, fresh water mostly.
• Body covered by bony dermal plates and scales (heavy armour), especially on the head shield.
• No jaws; mouth ventral; were filter or deposit feeders (mud suckers).
• Paired fins absent or poorly developed; tail heterocercal (often hypocercal).
• Internal skeleton cartilaginous; notochord persistent.
• Two semicircular canals in internal ear; median pineal eye present.

Examples

• Cephalaspis (Osteostraci) – flat head shield.
• Pteraspis (Heterostraci).
• Birkenia, Jamoytius (Anaspida).

Reasons of extinction

• Heavy armour made them slow; poor swimmers.
• No jaws, so limited to filter feeding; could not compete for food.
• Competition with and predation by the new jawed fishes (placoderms, sharks) in the Devonian.
• Climatic and habitat changes (drying of fresh waters).

Cyclostomes (lampreys and hagfishes) are believed to be their surviving relatives without armour.

Likely exam questions

1. (Long, 8 marks) Describe the digestive and respiratory system of Petromyzon with a labelled diagram.
2. (Long, 8 marks) Give the general characters of Vertebrata and discuss their affinities with Protochordata.
3. (Short, 4 marks) Describe the ammocoete larva and write its significance.
4. (Short, 4 marks) Write the general characters of Myxine.
5. (Short, 4 marks) Give the general characters of ostracoderms and the reasons of their extinction.

Syllabus topics (RTMNU)

• 3.1 Pisces: Salient features of Chondrichthyes and Osteichthyes.
• 3.2 General characters of Dipnoi: Neoceratodus, Protopterus Lepidosiren.
• 3.3 Labeo rohita: External features, Respiratory System, Urinogenital system.
• 3.4 Migration in fishes (Types and causes).

3.1 Pisces: salient features of Chondrichthyes and Osteichthyes

Pisces are cold-blooded aquatic vertebrates with jaws, paired fins, gills for respiration and a body usually covered with scales. Two main living classes are taught: Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony fishes).

Chondrichthyes (e.g. Scoliodon – dogfish shark, Pristis – sawfish, Torpedo – electric ray, Trygon – stingray, Chimaera)

• Mostly marine; endoskeleton of cartilage.
• Skin with minute placoid scales (dermal denticles).
• Mouth ventral; teeth are modified placoid scales.
• 5–7 pairs of gill slits, each opening separately; no operculum (except Chimaera).
• Tail heterocercal; no air bladder, so they must keep swimming to avoid sinking.
• Intestine with spiral valve; cloaca present.
• Males have claspers on pelvic fins; internal fertilisation; many are ovoviviparous or viviparous.
• Heart two-chambered with conus arteriosus.

Osteichthyes (e.g. Labeo, Catla, Hippocampus, Anabas, Exocoetus)

• Marine and fresh water; endoskeleton of bone.
• Skin with cycloid, ctenoid or ganoid scales.
• Mouth usually terminal.
• Four pairs of gills covered by a bony operculum; one gill opening on each side.
• Tail usually homocercal; air bladder (swim bladder) present for buoyancy.
• No claspers; external fertilisation; mostly oviparous.
• Heart two-chambered with bulbus arteriosus.

3.2 General characters of Dipnoi: Neoceratodus, Protopterus, Lepidosiren

Dipnoi (lungfishes; "double breathers") are bony fishes that breathe by gills and also by lungs (modified air bladder). They live in fresh waters that dry up in summer.

General characters

• Air bladder works as a lung; connected to the oesophagus ventrally.
• Internal nostrils (choanae) open into the mouth – air can be taken through the nose.
• Paired fins lobed (archipterygium) with a central jointed axis.
• Tail diphycercal; scales cycloid.
• Heart with partly divided atrium (incomplete inter-atrial septum) – a step towards double circulation.
• Notochord persistent; intestine has spiral valve.
• Larva of Protopterus and Lepidosiren has external gills, like amphibian tadpoles.

The three living genera

Significance: Dipnoi show many amphibian-like features (lungs, internal nares, three-chambered-like heart), so they are a link between fishes and amphibians, though amphibians are thought to come from the related crossopterygians.

3.3 Labeo rohita: external features, respiratory system, urinogenital system

Labeo rohita (Rohu) is a common Indian major carp of fresh water; bottom-column feeder on plants and detritus; important food fish.

External features

• Body spindle-shaped (fusiform), laterally compressed; divided into head, trunk and tail. Length up to about 1 m.
• Colour bluish/grey on back, silvery on sides and belly.
• Mouth sub-terminal with thick fringed lips; one pair of small maxillary barbels.
• Paired nostrils on the snout; large eyes without eyelids.
• Operculum covers the gills on each side.
• Body covered with cycloid scales; head without scales.
• Lateral line on each side (sense organ for water vibration).
• Fins: paired – pectoral and pelvic; unpaired – dorsal, anal and homocercal caudal fin. Fins supported by fin rays.
• Anus, genital and urinary openings just in front of the anal fin.

Respiratory system

• Four pairs of gills in the branchial (opercular) chambers, one on each gill arch. The fifth arch has no gill.
• Each gill (holobranch) has two rows of gill filaments (lamellae) with many secondary lamellae full of capillaries; inner side has gill rakers that protect the filaments and strain food.
• Mechanism: mouth opens, opercula close → water enters buccal cavity. Mouth closes, buccal floor rises → water flows over gills into opercular chamber → opercula open → water goes out.
• Blood in the lamellae flows opposite to water (counter-current flow) – this makes O₂ absorption efficient.
• Air bladder (two-chambered, connected to gut – physostomous) helps in buoyancy; not a respiratory organ in Labeo.

Urinogenital system

• Excretory: a pair of dark red, elongated mesonephric kidneys lie above the air bladder; anterior part is the head kidney (lymphoid/blood-forming). Ureters join to form a urinary bladder that opens by the urinary pore. Main waste is ammonia (ammonotelic).
• Male: a pair of elongated, creamy testes; sperm ducts unite and open by the genital pore.
• Female: a pair of large, yellowish ovaries full of eggs in the breeding season; oviducts open by the genital pore.
• Genital and urinary systems are separate in Labeo (not a true urinogenital duct). Fertilisation external; breeds in monsoon in flowing water (induced breeding by pituitary injection is used in fish farms).

3.4 Migration in fishes (types and causes)

Migration is a regular, mass, to-and-fro movement of fishes from one habitat to another, usually for breeding or feeding, and return.

Types

• Anadromous: from sea to river for breeding. Ex. salmon (Salmo), Hilsa (Tenualosa ilisha), Petromyzon.
• Catadromous: from river to sea for breeding. Ex. Anguilla (eel) migrates to the Sargasso Sea.
• Amphidromous: between sea and fresh water, not for breeding (for feeding/shelter). Ex. some gobies.
• Potamodromous: only within fresh water. Ex. carps, trout.
• Oceanodromous: only within the sea. Ex. tuna, cod, herring.
• Others: diurnal (vertical) migration – up at night and down by day; latitudinal – north–south.

Causes

• Breeding – to find safe spawning grounds for eggs and young (main cause).
• Food – search for rich feeding grounds.
• Climate – change of temperature, to escape cold or heat.
• Physico-chemical factors – salinity, oxygen, light, water current.
• Hormones – thyroid, pituitary and gonad hormones start the urge to migrate.
• Protection from enemies.

Orientation: salmon find their home stream by smell (olfactory imprinting); others use the sun, earth's magnetism and water currents.

Likely exam questions

1. (Long, 8 marks) Describe the external features and respiratory system of Labeo rohita with a labelled diagram.
2. (Long, 8 marks) What is migration in fishes? Describe its types and causes with examples.
3. (Short, 4 marks) Distinguish between Chondrichthyes and Osteichthyes.
4. (Short, 4 marks) Write the general characters of Dipnoi.
5. (Short, 4 marks) Describe the urinogenital system of Labeo rohita.

Syllabus topics (RTMNU)

• 4.1 Amphibia: Origin of Amphibia, affinities with Pisces.
• 4.2 Parental care and Neoteny in Amphibia.
• 4.3 Axolotl Larva, Hibernation and aestivation in Anura.
• 4.4 Frog (Rana tigrina): Circulatory system, Respiratory system.

4.1 Amphibia: origin of Amphibia; affinities with Pisces

Amphibia ("double life") are cold-blooded tetrapods that live on land but return to water to breed. They were the first vertebrates to come on land, in the Devonian period (about 370 million years ago).

Origin

• Amphibians came from lobe-finned fishes (Sarcopterygii), specially the Crossopterygii (Rhipidistia) like Eusthenopteron and Osteolepis.
• Evidence: crossopterygians had lungs, internal nostrils, lobed paired fins whose bones (one proximal, two middle, many distal) match the tetrapod limb (humerus, radius-ulna, carpals), and labyrinthodont teeth like early amphibians.
• Transitional fossils: Tiktaalik (fish with neck and wrist-like fin), Ichthyostega and Acanthostega (earliest tetrapods with limbs but a fish-like tail fin and lateral line).
• Why they came on land (Romer's theory): in the Devonian there were seasonal droughts; fishes with lobed fins and lungs could crawl from a drying pool to another pool. Land also offered food (insects, plants) and no enemies.

Affinities with Pisces

• Aquatic tadpole larva with gills, tail with fin, and lateral line system, like a fish.
• Eggs laid in water; external fertilisation (in most); eggs without amnion (anamniotes), like fishes.
• Skin is glandular and slimy.
• Heart of tadpole two-chambered; aortic arches like fish.
• Mesonephric kidney; 10 pairs of cranial nerves.

Differences: adult amphibians have limbs with digits instead of fins, lungs, three-chambered heart, middle ear with columella, eyelids, and no scales (except Ichthyophis).

4.2 Parental care and neoteny in Amphibia

Parental care

Definition: any care given by the parents to eggs or young to increase their survival. Amphibians lay many eggs, but some species lay fewer eggs and protect them.

• Nest building: Hyla faber (blacksmith frog) builds a mud-walled pool; Rhacophorus (tree frog) makes a foam nest on leaves above water.
• Carrying eggs on the back: Pipa americana (Surinam toad) – eggs sink into pits on the female's back skin and hatch as small toads; Gastrotheca (marsupial frog) carries eggs in a brood pouch on the back.
• Carrying eggs on legs: Alytes (midwife toad) – male winds egg strings around his hind legs until hatching.
• In vocal sac: Rhinoderma darwinii – male keeps eggs in his vocal sac until they become froglets.
• In stomach: Rheobatrachus (gastric-brooding frog, now extinct) – female swallowed eggs; young developed in the stomach.
• Guarding / coiling: Ichthyophis female coils around her eggs in a burrow; Dendrobates carries tadpoles on its back to water.
• Viviparity: Salamandra atra gives birth to young ones.

Significance: fewer eggs but higher survival; protects from predators and drying.

Neoteny

Definition: retention of larval characters in the adult, with the larva becoming sexually mature and breeding without metamorphosis. (When the larva itself reproduces it is also called paedogenesis.)

• Permanent (total) neoteny: larva never metamorphoses, even with thyroxine. Ex. Necturus, Proteus, Siren.
• Partial / temporary neoteny: larva metamorphoses if conditions change. Ex. axolotl larva of Ambystoma.
• Causes: low thyroid activity or low iodine in water; low temperature; tissues not responding to thyroxine; plenty of food in water but poor conditions on land; genetic factors.
• Significance: helps survival in harsh land conditions; neoteny is believed to have played a role in evolution of new groups (e.g. origin of chordates).

4.3 Axolotl larva; hibernation and aestivation in Anura

Axolotl larva

• Neotenic larva of the salamander Ambystoma (A. mexicanum / A. tigrinum), found in lakes of Mexico and USA.
• Has three pairs of feathery external gills, a laterally compressed tail with a fin, and a lateral line – larval characters.
• Becomes sexually mature and breeds in this larval form.
• When the pond dries, or when given thyroxine or iodine, it loses gills and tail fin, develops lungs and eyelids, and becomes the adult terrestrial Ambystoma.
• First shown experimentally that thyroid hormone controls amphibian metamorphosis.

Hibernation (winter sleep)

• Inactive, sleep-like state during cold winter. Frogs bury in mud at the bottom of ponds or under stones.
• Metabolism, heart rate and breathing fall very low; no feeding.
• Energy from stored glycogen in liver and fat bodies.
• Respiration only through the moist skin (cutaneous); lungs not used.

Aestivation (summer sleep)

• Inactive state during hot dry summer, to avoid heat and drying. Frogs/toads burrow into cool damp soil.
• Some form a cocoon of shed skin and mucus to reduce water loss (e.g. Cyclorana).
• Low metabolism; urea stored in body fluids; water stored in urinary bladder.
• Food reserve – glycogen and fat bodies; cutaneous respiration.

4.4 Frog (Rana tigrina): circulatory system, respiratory system

Rana tigrina (now Hoplobatrachus tigerinus), the Indian bull frog, is amphibious.

Circulatory system

• Closed type; blood, heart, arteries, veins; lymphatic system also present.
• Heart three-chambered: two auricles (right larger) and one ventricle. Accessory chambers: sinus venosus (dorsal) and truncus arteriosus (ventral) with a spiral valve.
• Working: deoxygenated blood from body → sinus venosus → right auricle. Oxygenated blood from lungs (pulmonary veins) → left auricle. Both auricles empty into the single ventricle → truncus. The spiral valve and spongy ventricle keep the two kinds of blood partly separate. So the heart has incomplete double circulation.
• Arterial system: truncus divides into right and left branches, each with three arches: carotid (to head), systemic (to body, they join to form the dorsal aorta), pulmo-cutaneous (to lungs and skin).
• Venous system: two precavals and one postcaval into sinus venosus; pulmonary veins into left auricle. Two portal systems: hepatic portal and renal portal.

Respiratory system

Frog uses three methods:

1. Cutaneous respiration: through moist, thin, vascular skin – in water, during hibernation and aestivation; also removes much CO₂.
2. Bucco-pharyngeal respiration: through the lining of the mouth; mouth closed, nostrils open, floor of buccal cavity moves up and down rapidly.
3. Pulmonary respiration: by a pair of thin, elastic, sac-like lungs with internal alveoli. Air passage: external nares → internal nares → bucco-pharyngeal cavity → glottis → laryngo-tracheal chamber → lungs.

Mechanism (buccal force pump, no diaphragm):

1. Inspiration: mouth closed, nostrils open; buccal floor lowered → air enters the buccal cavity.
2. Nostrils closed (by lifting the lower jaw) and buccal floor raised → air is pushed into the lungs through the glottis.
3. Expiration: elastic recoil of lungs and contraction of body wall muscles push air back to the buccal cavity and out through the nostrils.

Tadpole breathes by gills (first external, then internal).

Likely exam questions

1. (Long, 8 marks) Describe the structure and working of the heart of frog. Add a note on its arterial arches.
2. (Long, 8 marks) What is parental care? Describe parental care in Amphibia with suitable examples.
3. (Short, 4 marks) Write a note on neoteny and the axolotl larva.
4. (Short, 4 marks) Differentiate between hibernation and aestivation in Anura.
5. (Short, 4 marks) Describe the mechanism of pulmonary respiration in frog.