Characteristics of Phylum Chordata:
1. All chordates including the vertebrates have a flexible rod-like structure, the notochord (a simple pre-backbone structure), at some stage of their development.
2. They have a tubular dorsal nerve chord. This a bundle of nerves running along the “back” that splits into the brain and the spinal cord. It is hollow and lies dorsal to the notochord.
3. They have pharyngeal gill slits at some stage of their life histories. They are openings which allow the entry of water through the mouth without entering the digestive system. In many aquatic forms, they are lined with vascular lamellae, which form gills for respiration.
4. Post-anal tail: It is an extension of the body that runs past the anal opening.
5. The body has an organ system level of organisation.
6. They are true coelomates.
7. Bilaterally symmetrical, triploblastic, coelomic and segmented body.
8. A well-differentiated and complex body.
Note: The notochord is the defining structure of the chordates, and has essential roles in vertebrate development. In vertebrates a backbone or vertebral column replaces the notochord, therefore all vertebrates are chordates, but all chordates are not vertebrates.
Classification of Phylum Chordata:
Phylum Chordata is classified into three subphyla, namely;
1. Urochordata (tunicates)
2. Cephalochordata (lancelets)
3. Vertebrata (vertebrates)
All vertebrates have these features;
1. They are bilaterally symmetrical.
2. An internal skeleton or endoskeleton of bone and cartilage with a backbone or vertebral column made up of a series of small bones called vertebrae.
3. A well-developed central nervous system with a brain (within a brain case).
4. They have well-developed sense organs.
5. A closed blood circulatory system made up of a muscular heart.
6. The heart is three or four-chambered.
7. They have two pairs of limbs. The pair attached to the thoracic region is modified to form arms or wings, while the pelvic appendages have developed to form the pelvic fins, hind limbs or legs.
8. They have kidneys for eliminating body waste.
9. The skin of vertebrates may be naked or have a covering of scales, feathers or hairs.
10. They are triploblastic.
Classes of Vertebrates:
We have five major classes of vertebrates
a. Class: Pisces
b. Class: Amphibia
c. Class: Reptilia
d. Class: Aves
e. Class: Mammalia
a. Class Pisces (Fishes):
All fishes are aquatic. They are divided into subclasses.
i. The jawless Fishes: Agnatha like hagfish and lamprey. These are the most primitive. They have sucker-like mouth and no pair of fins.
ii. Chondrichthyes: Cartilaginous like sharks, skates, rays. They have jaws and pair of fins but their skeletons are made up of cartilage.
iii. Osteichthyes: They are bony fishes like tilapia fish, herrings. These are a diverse group of fishes made up of bones as opposed to cartilage as a component of the skeletal material. It is the largest class of vertebrates in existence today.
Characteristics of Fish:
1. They are cold-blooded or poikilothermic animals i.e the body temperature varies with that of their surroundings.
2. Their bodies are covered with scales (absent in jawless fishes) some are scaleless.
3. Gaseous exchange is by the gills. The gills are covered by opercula or operculum.
4. They have a two-chambered heart.
5. They are oviparous (i.e. they lay eggs with little or no embryonic development within the mother.) Fertilization is usually external.
6. A bony fish possess a swim bladder; a gas-filled organ that contributes to its ability to control its buoyancy and thus stay at its current water depth without wasting energy when swimming.
7. It has paired and unpaired fins for locomotion.
8. A dorsal fin is located on the top of a fish to staibilize it.
9. It has a well-developed sense of smell and a lateral line to detect vibration and changes in water pressures.
10. They possess homodont dentition.
b. Class Amphibia (Amphibians):
Amphibians include Frogs, Toads, Salamanders, and Newts. The class name is from the Greek word “amphi” which means dual “bios” which means life. This indicates that they live partly in water and partly on land. They were the first vertebrates to venture out of the water to live on land.
Characteristics of Amphibians:
1. They are poikilothermic (having a body temperature that varies with the temperature of the surroundings).
2. They possess moist, soft, and glandular skin.
3. Adults possess paired limbs with webbed toes for walking, hopping and swimming.
4. Respiration is by gills, lungs, skin, or mouth lining separately or in combination. Gills are present at the tadpole stage of its life cycle.
5. They have a three-chambered heart, two auricles and one ventricle.
6. They have two nostrils, eyes and often with movable eyelids, eardrum (external), mouth.
7. Fertilization is external and they are oviparous i.e. they lay eggs with yolks.
8. Tadpoles are herbivorous, eating algae and decaying matter. As adults, amphibians are carnivores, eating insects, worms and occasionally small vertebrates.
9. They have sticky tongues which can be protruded and retracted quickly.
10. Examples of amphibians are frogs, African toad (Bufo regulars), salamanders, and newts.
c. Class Reptilia (Reptiles)
Reptiles are believed to have evolved from amphibians. They are the first group of vertebrates adapted to terrestrial life.
Characteristics of Reptiles:
1. They possess dry and scaly bodies.
2. Most possess two pairs of limbs with five toes ending with claws and are suited for running, crawling, and climbing while others e.g. snakes have no limbs.
3. Dentition is homodont.
4. They are poikilothermic.
5. They respire with the aid of the lungs.
6. Fertilization is internal with the laying of soft and leathery-shelled eggs. Most are oviparous (animals that lay eggs with little or no embryonic development) while few others are ovoviviparous (producing eggs that develop in their maternal body until they are ready to hatch.)
7. It has an incomplete developed four-chambered heart e.g. lizards, snakes, crocodiles etc.
Advantages of reptiles over amphibians are;
i. Dry and scaly body covering.
ii. A more developed four chambered heart.
iii. Eggs suited for development on land.
iv. Internal fertilization.
v. Presence of claw digits.
d. Class Aves (Birds)
Characteristics of Birds:
1. Their body is covered with feathers (made up of keratin protein) except for the feet which are covered with scales.
2. Their forelimbs are modified into wings for flying while hind limbs are adapted for perching, walking, or swimming. Each foot has four toes ending in claws.
3. The mouth is modified into a horny toothless beak with no teeth.
4. They are homeothermic i.e. able to keep their body temperature more or less constant.
5. Its skeleton is rigid and the bones are hollow and light with air sac which enables them to fly.
6. Respiration is by the lungs which contain thin-walled extensions called air sacs.
7. The heart is four-chambered with two auricles and two ventricles.
8. Fertilization is internal and they lay hard-shelled eggs with large yolks (oviparous)
9. Members can be divided into two groups, namely, flying birds like parrots, pigeons, waver birds etc., and flightless birds like ostrich, kiwi, Domestic fowls etc.
Advancement of birds over other groups are:
i. Have on insulted body covering feather.
ii. Regulated body temperature.
iii. Ability to fly.
iv. Four chambered heart.
v. Specialized care for the young.
e. Class Mammalia:
They are advanced animals
Characteristics of Mammals:
1. The body has an exoskeleton made up of skin completely or partly covered with hair (or fur).
2. Possess mammary gland which secretes milk used for nourishing the young after birth.
3. They possess an external ear (pinna).
4. Dentition is heterodont i.e. different types of teeth with different shapes, sizes and functions.
5. Two sets of teeth occur during a lifetime of most mammals, a milky set and a permanent set (diphyodont condition) e.g. Homo sapiens. There are some with monophyodont dentition (only one set of teeth during lifetime) e.g. Moles and Squirrels.
6. They possess sweat and sebaceous glands.
7. They have diaphragm (breathing muscles) separating the thoracic cavity from the abdominal cavity.
8. They have a well-developed brain.
9. They are homeothermic i.e they have a constant body temperature.
10. Respiration is by lungs.
11. They have eyes and moveable eyelids.
12. They have four-chambered hearts, two auricles and two distinct ventricles.
13. Most possess four limbs for locomotion.
14. They carry out sexual reproduction, sexes are separate.
15. They are viviparous, giving birth to young ones alive.
16. Some are secondarily adapted for living in water – Whales.
17. Examples include Rats, Rabbits, Man etc.