Phylum Chordata

1. They have a notochord at some stages of their development. Notochord is a stiff, flexible rod located dorsal to the gut and ventral to the nerve cord. In most chordates the notochord is replaced by a vertebral column (backbone) during adulthood.
2. They have visceral clefts which are slits perforating the body wall in the pharynx. They are useful in fishes and tadpole larva of amphibians for gaseous exchange as they use gills. In adult amphibians, reptiles, aves and mammals, the visceral clefts never develop gills.
3. They have endoskeletons made up of bone and cartilage.
4. They have post-anal tail.
5. They have a dorsal hollow nerve cord found above the notochord and below the epidermis. In higher chordates, the anterior part forms a brain and the posterior part forms a spinal cord.

There are six classes of chordates namely; Chondrichthyes (cartilaginous fish such as shark and rays), Osteichthyes (bony fish such as tilapia and tuna), Amphibia (amphibians such as frog and toad), Reptilia (reptiles such as lizard, crocodile, and snake), Aves (birds such as hen, parrot, eagle, and pigeon) and Mammalia (mammals such as human, mouse, bat, rabbit and monkey).

All members of this class have paired fins, paired nares, jaws, and a two chambered heart. Additionally, they have scales and endoskeletons made of cartilage. Members of class Chondrichthyes include dogfish, sharks, skates, and rays.

Members of the class Chondrichthyes have the following unique features, which distinguish them from other fishes.

1. They possess placoid scales (tooth- like) covering their skin.
2. They have cartilaginous endoskeleton.
3. They have five pairs of visceral clefts as separate gill slits for gaseous exchange, with no opercula or gill covers.
4. They have ventral mouths.
5. They have heterocercal tail fins (asymmetric in shape), which prevent them from sinking as they lack swim bladder for buoyancy.
6. They have paired fleshy pectoral and pelvic fins for swimming.
7. They undergo internal fertilisation.

The body is more or less completely covered by placoid scales and has a lateral line that runs along the side of its body for detecting water currents and wave intensity. Its pectoral fin is anterior to the pelvic fins; usually the latter has claspers (pterygopodes) which are used as copulatory organs in males, and the caudal fin is heterocercal in shape. The mouth is located ventrally with teeth, which are constantly replaced. It has small eyes, which have lower lids used to cover the eyes during feeding.

The gill slits are naked (no operculum) and has a spiracle which is a modified gill slit on top of the head that provides oxygenated blood directly to the eye and brain through a separate blood vessel. The intestine is short but with absorptive features increased by spiral valves.

The class includes all bony fishes, such as tuna, tilapia, rainbow fish, herring and catfish. They are all aquatic organisms found in marine and fresh water bodies.

The following features differentiate members of the class Osteichthyes from other fishes.

1. They have thin, round, and cycloid scales (made up of bones) on their skins, which are impermeable to water, hence reducing water loss in marine environment, and restrict entry of water in fresh water fish.
2. They are bony fish, as they have bony skeletons.
3. They have paired pectoral and pelvic fins, supported by bony rays for swimming.
4. They have homocercal (symmetrical) tail fins.
5. They have four pairs of visceral clefts as separate gill openings covered by opercula.
6. They produce eggs and undergo external fertilisation.
7. They have swim bladder, which is in the abdominal cavity next to the backbone. This helps them to rise and sink (buoyancy) in water at particular depth without using much energy.
8. They have a terminal mouth.

The body is laterally flattened and tapered at both ends. Has a streamlined shape to overcome water resistance during swimming. Its body surface is covered by cycloid scales, which point backwards in order to reduce resistance during swimming. The fish has a lateral line that runs along the side of its body.

The lateral line is a series of sensory organs called neuromasts that helps the fish to sense vibrations and water pressure for navigating and locating prey

This class includes amphibians such as frogs, toads, caecilians as well as salamanders. Amphibians are tetrapod with the exception of caecilians, which are limbless. All amphibians lay eggs with the exception of some caecilians and some toads. For example members of the caecelian genera Scolecomorphus and Schistometopum give birth to live young.

Also the toad genus Nectophrynoides; the members of this genus also give birth to live young. In the latter genus of Nectophrynoides there is the Kihansi Spray Toad (Nectophrynoides asperginis); a small toad endemic to Tanzania with this unique characteristic of giving birth to live young toadlets.

The word amphibia refers to "double life", or life in water and on land. Amphibians are cold-blooded (ectothermic) vertebrates whose body temperature is not regulated by internal mechanisms. They inhabit a wide range of habitats, ranging from terrestrial to aquatic environments.

Most amphibians spend a part of their life in aquatic environment and another part in terrestrial environment. Due to their life cycle and physiology, many amphibians are "tied" to water bodies in order to survive and reproduce. Normally, amphibians undergo metamorphosis from a juvenile as an aquatic larval form into a terrestrial adult.

Amphibians have the following distinctive features:

1. They dwell both in water and on land, as they depend on water for reproduction, hence the name amphibian, which means double life.
2. Amphibians' offspring begin their life cycle under water and breath by means of gills. As they grow to adulthood they move to terrestrial and breathe by means of either skin or lungs.
3. They have soft moist skin without scale, used for gaseous exchange to supplement lungs and buccal cavity.
4. Amphibian eggs have a jelly structure, which is prone to dehydration when exposed to air.
5. Amphibian's fertilisation takes place outside the female body (with the exception of some caecilians and some toads including the Kihansi spray toad which give birth to live young).

The body of the adult frog is divisible into the head and trunk. The neck and tail parts are absent. The head is blunt, and the mouth is terminal, with two flexibly movable jaws. The head bears external nares, a pair of nostrils that lead into nasal passages. The eyes are large, bulged and covered by a nictitating membrane that protects them while in water. Behind the eye, there is a circular patch, a tympanum or eardrum, which receives sound signals.

The class comprises a group of animals including turtles, crocodiles, alligators, chameleons, tortoises, snakes, and lizards. Most of them have four limbs while others do not have limbs. Reptiles are found in diverse habitats, such as deserts, mountains, rocks, treetops and in water. They are mostly terrestrial with few aquatic members such as turtles and terrapins. They are cold-blooded (ectothermic) vertebrates. Their body temperature fluctuates according to the environmental temperature. Reptiles regulate their body temperature by behaviour, either by basking in the sun to warm themselves or by hiding under cover to cool their body. Most of them are tetrapods, with claws on their toes. Reptiles such as snakes and some lizards are legless, although they are descendants of four-limbed ancestors.

Reptiles possess the following unique features, which differentiate them from other chordates:

1. They have dry scaly skin with horny scales.
2. They have two pairs of pentadactyl limbs, except some members, such as snakes, which have no limbs.
3. They undergo internal fertilisation and their fertilised yolky eggs are laid on land or retained until hatching
4. They lay amniotic eggs that have a leathery shell to prevent rapid waterloss.
5. Their eyes are located at the front of the head to facilitate binocular vision. Some reptiles can move each eye independently, and this helps them to find food and escape from predators.

Lizards have dry scaly skin, and most of them have clawed feet and external ear openings. Most lizards are small, with four legs and a long tail that, in many species, is fragile and easily broken but regenerate later. The legs of some lizards are greatly shortened or vestigial, making animals such as the glass lizard or a slow worm have a snake-like appearance. They are distinguished from true snakes by their movable eyelids.

The class Aves includes all birds. It is an extremely distinctive and successful class. Aves are bipedal feathered and warm-blooded (homoeothermous) animals, as they are able to maintain a constant body temperature. They have unique "one-way" breathing system. They have light, yet strong hollow bones, forming a skeleton in which many bones are fused or lost and have powerful flight muscles. Birds have evolved specific adaptations to enable them fly. They have fused hollow bones making birds have light weight, have a large keel for attaching flight muscles and have large chest muscles used for flight.

Birds also have their fore limbs modified for flight and have feathers which are used for flying. They have also developed long flight feathers on the wings and tail to help birds attain balance and steer. Birds have developed air sacs connected to the lungs enabling them to extract more oxygen to release more energy to power flight. Also they have four chambered heart that enable them get more oxygen and avoid mixing oxygenated and deoxygenated blood. Birds have well developed brain enabling them to have quick reaction during flight. They have large eye to body ratio; and large eyes give birds good and keen eyesight important in flight. Birds can migrate during harsh conditions, since they have wings that enable them to move fast. Birds are widely spread all over the world; some are found in very cold snowy environments and others in dry and hot environments.

The body is spindle shaped and the size varies from 20-25 cm. The body is divided into the head, neck, trunk, and tail. Most parts of the body are covered by feathers. The head is small and rounded. It is anteriorly pointed into a short beak. On the lateral side of the head, there is a pair of prominent eyes. The ear comprises of small apertures on the posterior side of the eyes. Each aperture remains covered by a special group of feathers called auricular feathers. Each aperture leads to a canal called external auditory meatus, which is closed below by the tympanic membrane. The trunk is the greatest and widest part of the body. It is boat-shaped and bears a pair of wings and a pair of legs. The entire foot is covered with horny epidermal scales. At the hind end of the trunk is the cloaca aperture

Aves are different from other chordates due to possession of the following features:

1. Their bodies are covered by overlapping feathers.
2. They lost teeth; instead, they have modified mouthparts into different types of beaks (bills). Beaks are adapted for many different feeding habits, such as seed crushing, fruit scooping, flesh tearing, nectar sipping, and wood chiseling.
3. They have cylindrical, long necks connecting heads and trunks.
4. They have two pairs of pentadactyl limbs. The front limbs are modified into a pair of wings, which bear quill feathers for flight, while the hind limbs are covered by scales and are adapted for perching, walking or swimming.
5. They undergo internal fertilisation and produce eggs with calcareous shells.
6. Their sternum forms a sharp ventral keel, providing muscles attachment for flight.

The class Mammalia consists of all animals with mammary glands. It is an extremely diverse and very advanced group in the kingdom Animalia. Members of the class Mammalia include: human, mouse, rabbit, cow, lion, bat, whale, and donkey.

Mammals are different from other chordates because:

1. They have fur or hair that cover their skin. The skin is glandular, with two types of glands, namely, sebaceous and sweat glands.
2. Females (mothers) have mammary glands, which produce milk for their newborn.
3. They have external ears called pinna in addition to middle and inner ears, which are used for collection of sound waves and leading them to ear canal. The middle ear has three small soft bones called ear ossicles namely; malleus, incus, and stapes.
4. They have heterotrophic mode of nutrition with different types of teeth (heterodont dentition) for different functions, depending on the mode of feeding.
5. They have highly developed brains.
6. They have muscular diaphragm, which separates the thorax from abdominal cavity.
7. They are viviparous (give birth to young ones). The developing foetus is held in the uterus, and gets nourishment through placenta, except in primitive animals like kangaroo, which bear immature young ones and duck-billed platypus, and echidna which lay eggs.

Structurally, an adult mouse has an average length of 7.5-10 cm. Its body is divided into head, neck, and trunk. The head bears external ear flaps called pinnae, eyes, nostrils and a mouth. The latter has long, hair extensions called vibrissae or tactile whiskers. The neck is short and wide, connecting the head to the trunk. The trunk bears four walking legs, two short hind legs and two long front legs, each with five digits. The trunk also bears a long tail which is either hairless or has sparse covering of hair. The whole body of the mammal is covered by fur (pelage).