CLASSIFICATION OF ANIMALS

The animal kingdom is divided into invertebrates (animals without backbones) and vertebrates (animals with backbones).

 

Invertebrates

The subphylum invertebrate (the invertebrates is made up mainly of the following phyla: Protozoa, Coelenterata, Platyhelminthes, Nematoda, Annelida, Mollusca, Echinodermata and Arthropoda.

 

i) Phylum: Protozoa

Characteristic features

1. They are one-celled, microscopic animals or colony of one-celled animals.
2. They are either free-living or parasitic organisms.
3. They are either aquatic or endoparasitic in habitat. Examples are Amoeba, Paramecium, Plasmodium, Trypanosome and Volvox.

 

ii) Phylum: Coelenterata

Characteristic features

1. They are multicellular organisms.
2. They have two distinct body layers (diploblastic), ectoderm (outer layer) and endoderm (inner layer), separated by a featureless middle layer called mesogloea.
3. They have only one body cavity called enteron.
4. There is only one opening called mouth to the outside world. No anus.
5. Mouth surrounded with tentacles.
6. Possession of stinging cells called nematocysts
7. They have specialized cells such as glandular, nerve, sensory and absorptive cells.
8. Asexual reproduction is by budding. Hydra, Obelia, sea anemone, jelly fish, coral and Physalia are examples.

 

iii) Phylum: Platyhelminthes

Characteristic features

1. Soft, flat and unsegmented body.
2. They are bilaterally symmetrical.
3. They have no body cavity (acoelomate).
4. They possess alimentary canal with highly branched diverticula. No anus.
5. Some are free living e.g Planaria, while others are parasitic e.g liver flukes and tapeworms.

 

iv) Phylum: Nematoda (Roundworms)

Characteristic features

1. They have soft, cylindrical and slender bodies pointed at both ends.
2. Body is not segmented.
3. They have alimentary canal with both mouth and anus.
4. They are bilaterally symmetrical.
5. They have false body cavity (Pseudocoelomates).
6. They are mostly parasitic but few are free living. Examples are Ascaris (roundworms), hookworms, threadworms, guinea worms and vinegar eel (free living).

 

v) Phylum: Annelida

Characteristic features

1. The body is segmented with each segment separated from other by septa.
2. Body is cylindrical in shape.
3. They have alimentary canal with mouth and anus.
4. They possess true body cavity (coeloma) with a distinct head.
5. The appendages (setae) are not jointed. Examples are earthworms, tubeworms and leech.

 

vi) Phylum: Mollusca

Characteristic features

1. They possess soft and unsegmented bodies.
2. Tentacles are present in most members.
3. The bodies of most of them are covered with shells e.g. land snail, (Archatina) while others are not e.g. octopus and squid.

 

vii) Phylum: Echinodermata

Characteristic features

1. They possess tough spiny and calcareous exoskeleton.
2. They are radially symmetrical in adults but bilaterally symmetrical in larvae.
3. The head is not usually distinct.
4. Most of them are star shaped.
5. They are all marine organisms.

Examples are bristle star, starfish, sea urchin, sea cucumber and sea lily.

 

viii) Phylum: Arthropoda

Characteristic features

1. Their bodies are covered with hard exoskeleton of chitin.
2. Their appendages are jointed.
3. Often the bodies are divided into three regions – head, thorax and abdomen. In some, the head and thorax are fused together to form a cephalothorax as in crustaceans and arachnids.
4. They have bilateral symmetry.

Examples are insects (mosquito and cockroach), crustaceans (e.g. crab and lobster), diplopods(e.g millipede), chilopods (e.g centipede) and arachnids (e.g spider and tick).

 

 

Phylum: Vertebrate (the vertebrates)

The phylum vertebrates consists of animals with backbones belonging to five classes namely:

Class: Vertebrates are represented by five classes namely: Pisces (fish), Amphibia, Reptilia, Aves (birds) and Mammalia.

 

i) Class: Pisces (Fishes)

Characteristic features

1. They are all aquatic.
2. They possess gills for respiration throughout life.
3. One-looped circulation, that is blood is pumped from the heart to the gills where oxygenated blood is passed to the rest of the body and returns to the heart.
4. They possess fins for locomotion.
5. Their bodies are streamlined in shape.
6. They are cold blooded (poikilothermic).
7. Fertilization is external.
8. Most of them have scales (e.g. Tilapia) while others are scaleless (e.g.Clarias).

Examples are Tilapia, Hemichromis, Clarias, electric cat fish, Gymnarchus and shark.

 

The Class Pisces is divided into two subclasses namely: Chondrichthyes and Osteichtyes

 

a) Subclass: Chondricthyes

Characteristic features

1. Skeleton is made of cartilage only.
2. Mouth is inferior, that is, it is ventrally.
3. Fins are fleshy.
4. Each gill has its own opening
5. Generally, the upper lobe of the caudal tail is longer.
6. Possess placoid scales, that is, scales are tooth-like in structure.

 

b) Subclass: Osteichthyes

Characteristic features

1. Skeleton is bony.
2. Mouth is either placed anteriorly (on top of snout), terminally (front of snout), ventrally (below snout). Mouth is protrusible and retractable, that is, mouth can be pushed forward and withdrawn.
3. All gills are covered with a bony plate .
4. Fins are supported by bony or soft rays.
5. Scales when present are either cycloid (margin is smooth and rounded) ctenoid (exposed posterior).

 

ii) Class: Amphibia

Characteristic features

1. Possess moist, soft and scaleless skin.
2. They are cold blooded (poikilothermic).
3. Adults possess paired limbs with webbed toes.
4. The larval stage is aquatic while the adult is either aquatic or terrestrial. Tadpole larva is vegetarian while adult is usually carnivorous.
5. The heart is two chambered in larval stage and three chambered in adult.
6. Fertilization is external.
7. Breathing is by gills in the larval stage and by lungs, skin and mouth in the adult stage. Examples are toad, frog and newt.

 

iii) Class: Reptilia

Characteristic features

1. They possess dry and scaly skin.
2. Most possess four limbs which end in claws while others (snake) have no limbs.
3. Dentition is homodont (the same type of teeth).
4. They are cold blooded (poikilothermic).
5. Breathing is by lungs.
6. Fertilization is internal with laying of soft and leathery shelled eggs. Most members are oviparous while a few others retain their eggs in their bodies until hatching (ovoviviparous).
7. They are mostly terrestrial but few such as turtles and crocodiles are aquatic. Examples are lizard, snake, crocodile, tortoise and chameleon.
8. The heart is three chambered except crocodile with four chambered heart.

 

iv) Class: Aves

Characteristic features

1. The body is covered with feathers except the legs and the feet, which are covered with scales.
2. Their forelimbs are modified as wings.
3. The mouth is modified into horny toothless beak or bill.
4. Their bones are porous, light and in some cases filled with air.
5. Body is supported by two hind limbs, (bipedal stance).
6. They are warm blooded (homeothermic). They are very active animals.
7. Breathing is by lungs.
8. They lay hard shelled eggs (oviparous).
9. Fertilization is internal.
10. They are mostly terrestrial.
11. Heart is four chambered.

Examples are domestic fowl, duck, eagle and pigeon.

 

v) Class: Mammalia

Characteristic features

1. They possess hairs. However, some mammals such as whales have no hairs while pangolins have scales instead of hairs.
2. They possess mammary glands. The mother’s mammary glands secrete milk used in nourishing the young after birth.
3. They possess external ears.
4. All mammals have three tiny bones in their ears namely: hammer (molleus), anvil (incus) and stapes (stirrup).
5. They are viviparous; young are nourished during development in the mother’s uterus and born at an advanced stage of development. However, the monotremes lay eggs which hatch into young nourished by milk secreted by mammary glands of the mother.
6. Breathing is by lungs.
7. They have diaphragm (breathing muscle) cavity.separating the thoracic cavity from the abdominal cavity.
8. Mammalian heart is four chambered only left aortic arch.
9. Most possess four limbs for locomotion.
10. They are warm blooded (homeothermic) hence they are very active animals.
11. Most species possess seven cervical (neck) bones.
12. Possess two pairs of limbs usually for locomotion.
13. The cerebrum and cerebellum of the brain are highest developed than other animal.

Examples are duck-billed platypus, kangaroo, dog, cat, pangolin, elephant, giraffe, monkey, gorilla, hippopotamus, zebra, cane rat, lion, horse, baboon, porcupine and man.