Phylum Chordata (Class - Amphibia & Reptilia) - NEET PDF Download

Amphibia

The organisms belonging to the class Amphibia fall under the Chordata phylum of the kingdom Animalia. These are multicellular vertebrates that live both on land and water. This class includes about 3000 species. They are the first cold-blooded animals to have appeared on land.

Class Amphibia

Characteristics of Class Amphibia

The characteristics of the organisms present in class amphibia are as follows:

• These can live both on land and in water.
• They are ectothermic animals, found in a warm environment.
• Their body is divided into head and trunk. The tail may or may not be present.
• The skin is smooth and rough without any scales, but with glands that make it moist.
• They have no paired fins. Unpaired fins might be present.
• They have two pairs of limbs for locomotion.
• They respire through the lungs and skin. Gills might be present externally in some adults.
• The heart is three chambered.
• The kidneys are mesonephric. The excretory material includes ammonia and urea.
• They possess ten pairs of cranial nerves.
• The lateral line is present during their development.
• The sexes are separate and fertilization is usually external. However, in salamanders, the fertilization is internal.
• Development is indirect with metamorphosis.
• Breeding occurs in water. The copulatory organs are absent in males.
• Eg., Frogs, Salamanders.

Classification of Amphibia

The Amphibians are divided into three orders. The classification of amphibia is given below:
➤ Apoda (Gymnophiona or Caecilia)

• Apoda means “without legs”.These are limbless organisms with scales on their body.
• They are also known as “blind-worms” because their eyes are covered by skin or bone.
• The tentacles on their head are the chemosensory organs that help them to detect the underground prey. Eg., Caecilians
• They possess venom glands.
• They secrete mucus to reduce water loss.

➤ Urodela (Caudata)

• These are the organisms with a tail.
• The body is elongated with four equally sized limbs.
• The skin is smooth with poison glands.
• Fertilization is internal.
• They feed on insects and worms. Eg., Salamanders
• They are found under leaf litter, in the soil, or in water.
• In the southern US, they reproduce primarily in winters.
• Very little differences between male and female.
• Spermatophores are utilized for internal fertilization.
• They possess hidden gills.

➤ Anura (Salientia)

• There are around 3400 species of Anura in the world.
• They have four limbs. The front limbs are elongated and modified to jump.
• The head and trunk are fused together.
• The tail is present only in the larval stage and is lost in the adults.
• Fertilization is external and the eggs are laid in water. Eg., frogs and toads.

Reptilia

These were the first class of organisms to adapt to life on land. They are believed to have evolved from the amphibians millions of years ago. There are about 10000 different species of reptiles on earth. They are cold-blooded animals belonging to the phylum Chordata of Animal kingdom.
The skull of the reptiles is modified that gives the reptiles an efficient and powerful jaw action. The modification also makes the skull light.

Characteristics of Reptilia

Following are the important characteristics of the animals belonging to Class Reptilia:

• These are creeping and burrowing terrestrial animals with scales on their body.
• They are cold-blooded animals found in most of the warmer regions of the world.
• Their skin is dry, and rough, without any glands.
• The body is divided into head, neck, trunk, and tail.
• Few of these shed the scales on their skin as skin cast.
• The respiration takes place with the help of the lungs.
• The skull is monocondylic.
• They have two pairs of pentadactyl limbs, each bearing claws. Snakes are an exception.
• The heart is 3 chambered. However, crocodiles have a 4-chambered heart.
• The nervous system comprises of 12 pairs of cranial nerves.
• The lateral line system is absent in reptiles.
• Except for snakes, all the reptiles have well-developed ears.
• They possess a typical cloaca.
• Reptiles are ureotelic, uricotelic, and ammonotelic.
• Fertilization is internal.
• They exhibit a meroblastic segmentation.
• They are oviparous and the eggs are very yolky.
• Eg., Snakes, Turtles, Lizards, Crocodiles

Classification of Reptilia

The class Reptilia is differentiated into two major sub-classes:

• Anapsida
• Parapsida
• Diapsida

➤ Anapsida

• The dermal bones form a complete roof over the skull with no temporal fossae.
• These are sub-divided into Cotylosauria and Chelonia.
• Modern chelonians are classified according to the method of retracting the head in the shell.
• Turtles, tortoises, and terrapins belong to this group.

➤ Parapsida

• These reptiles possess one temporal fossa present high up on the skull.
• Protosaurs, Nothosaurs, Placodonts showed this type of skull.
• The two largest groups among these were- Ichthyosaurs and Plesiosaurus. These became extinct at the end of the Cretaceous period when several other reptiles including dinosaurs died.

➤ Diapsida

• There are two temporal vacuities in the skull.
• They are diverse of all reptiles.
• The dinosaurs and pterosaurs are included in this group.
• These are divided into two major groups- Archosauria and Lepidosauria.
• Eg., Crocodilus, Chameleon

Groups of Reptiles

The class Reptilia is further divided into different groups known as orders:

➤ Extinct groups of class Reptilia

• Ichthyopterygia
• Lepidosauria
• Archosauria
• Synapsida

➤ Ichthyopterygia

• This group had one temporal fossa place high up on the skull.
• The two largest groups belonging to this sub-class are Ichthyosaurs and Plesiosaurs.
• Eg., Ichthyosaurus

➤ Lepidosauria

• These were all the lizard-like reptiles.
• They had two temporal vacuities in the skull.
• Eg., Youngina,

➤ Archosauria

• The skulls were diapsid.
• Some were bipedal which gave rise to birds.
• They also gave rise to dinosaurs.
• Eg., Brontosaurus

➤ Synapsida

• They had one temporal fossa on the lower side of the skull.
• These were the most dominant group of reptiles during the Permian period.
• The surviving Lepidosaurs in the Mesozoic era gave rise to mammals. The rest were replaced by dinosaurs.
• Eg., Plesiosaurus.