Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Vertebrata: Amphibia to Mammalia.

A frog lives on land, yet it must go back to water to lay its eggs. Reptiles got past that barrier, and that is how they spread so fast over the earth. The story of the four-legged vertebrates is the story of that barrier being crossed, and at the end birds and mammals arrive at the same kind of heart.

1. Amphibia

Amphibia is a Greek name: amphi means both, bios means life. This was the first class to set out on land on four legs, so its members are tetrapods. They are mainly land animals, but at breeding time they must come to water to lay eggs, because the eggs develop in water. That is why they are called amphibious. There are 7,700 living species: 715 tailed, 200 limbless, and the remaining 6,785 four-legged, such as frogs.

Amphibians are ectothermic tetrapod vertebrates with glandular skin. The larva is aquatic; the adult is aquatic or terrestrial. The skin is smooth and moist, and it helps in respiration. There are four clawless digits on the forelimb and five on the hindlimb. The larva breathes by gills. The adult breathes by lungs, skin and the buccal mucous membrane. The heart is three-chambered: two atria and one ventricle.

Examples: Salamandra salamandra (salamander), Rhacophorus fergusonii (flying frog), Hoplobatrachus tigerinus (bull frog), Fejervarya asmati (Asmati frog), Chiromantis simus (tree frog).

2. Reptilia

Reptilia comes from the Latin repto, to creep. They overcame the structural and physiological barriers of amphibians, such as dryness and egg development, and spread over the earth. There are 10,450 living species. The body is dry and covered by epidermal scales or hard plates. Each foot has five clawed digits.

The ventricle is incompletely divided, so the heart is incompletely four-chambered. The crocodile is the exception, with a completely four-chambered heart. The lungs are the only respiratory organ. The egg is covered by a leathery or calcareous shell. Extra-embryonic membranes form during development, so there is no larval stage.

Examples: Gavialis gangeticus (gharial), Lissemys punctata (tortoise), Naja naja (cobra), Kachuga sylhetensis (roofed turtle), Hemidactylus frenatus (house lizard).

3. Aves

Aves is Latin avis, a bird. This vertebrate group left water and land for the sky, and has more than 10,000 species. Every change aims at one thing: lightening the body so that flight succeeds. The body is covered with feathers, the neck is long and S-shaped, and the forelimbs are modified into wings. The jaws have become a toothless beak. The bones are pneumatic and light, and many are fused.

Thin air sacs join the lungs and even enter the bones. An efficient digestive system supports rapid metabolism. The heart is four-chambered, with two atria and two ventricles. The warm-blooded (endothermic) state first appeared in birds.

Examples: Corvus splendens (crow), Columba livia (pigeon), Copsychus saularis (magpie robin), Passer domesticus (sparrow), Francolinus gularis (francolin), Centropus bengalensis (lesser coucal).

4. Mammalia

Mammalia is Latin mamma, a breast. It is the most advanced animal group, from the 1.5 g bat to the 130-tonne blue whale, with about 6,000 species. Humans belong to this class. The skin has glands (sweat, sebaceous and others) and, except in whales, is covered with hair. The newborn is nourished by milk from the mammary glands of the mature female.

The outer ear has a pinna and the middle ear has three small bones. The jaws bear different kinds of teeth. A muscular diaphragm separates the thoracic and abdominal cavities. Mature red blood cells have no nucleus. The heart is completely four-chambered and the animal is endothermic. One subgroup, the bats, can fly.

Examples: Ornithorhynchus anatinus (platypus), Macropus rufa (kangaroo), Pteropus giganteus (bat), Balaenoptera musculus (blue whale), Panthera leo (lion), Panthera tigris (tiger), Dicerorhinus sumatrensis (two-horned rhinoceros), Homo sapiens (human).

5. Match features and animals

Now take the four classes in hand. For each feature, ask: what is the skin like, how many chambers in the heart, what about the egg or larva?

Now the names. Read the scientific name and say the class.

6. Vertebrates and invertebrates

A vertebrate has a vertebral column of vertebrae along the dorsal midline; an invertebrate has none. The vertebrate nerve cord is hollow and dorsal; the invertebrate one is solid and ventral. Vertebrates are bilaterally symmetrical; invertebrates are asymmetrical, radial or bilateral. The vertebrate heart is ventral; the invertebrate heart, if present, is dorsal.

Vertebrate haemoglobin is always in the red cells; invertebrate haemoglobin, if present, is dissolved in the plasma. The vertebrate brain is always present and protected in a skull; an invertebrate brain, if present, is not covered by a skull. The vertebrate endoskeleton is made of bone and cartilage; invertebrates have no endoskeleton, or one not made of bone and cartilage.

7. Five vertebrate groups side by side

The book's table has ten rows. To fit a phone screen I have cut it into four pieces, each with the five groups as rows. Tap any cell and the full answer for all five groups on that feature appears below. First skin, movement and breathing.

Now the heart, the blood and the cranial nerves.

Then the eyelids and the cloaca.

The last grid is about reproduction: where fertilisation happens, and what is born.

Climb the heart ladder from fish to mammal. At each rung, watch the wall between the chambers: none, half, whole.

The heart row is the one asked most often. Put the five groups in order, from the simplest heart to the most advanced.

8. Chordate and vertebrate are not the same

All vertebrates are chordates, because every chordate feature is present in their embryo. But not all chordates are vertebrates: in Urochordata and Cephalochordata the notochord is never replaced by a vertebral column.

The three distinctive chordate features are an elastic notochord, a dorsal hollow nerve cord and pharyngeal gill slits. In Protochordata they last for life. In Vertebrata the notochord is replaced by the backbone, the nerve cord by the brain and spinal cord, and the gill slits close as gills or lungs appear.

9. Check yourself

Start with the first question.

In the second, recall how the groups differ.

The last is in the exam's style.

The whole note on seven cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Vertebrata: Amphibia to Mammalia” part.