If we look at these animals’ body carefully, we can divide them into two categories. Some animals have a backbones whereas others do not have it. Animals like cows, dogs and crows have backbones. They have a skeleton made from bones and cartilages. Snails, houseflies and mosquitoes and not have backbones. The animals that have a backbone are called vertebrates. Thus, cows, dogs and crows are vertebrates. About 2% of the total animals are vertebrates. There are about 57,739 species of vertebrates. Besides having a backbone, there are other characteristics of vertebrates. Those characteristics of vertebrates are:
- They have well developed sense organs and a brain.
- They have an endoskeleton.
- They usually have paired appendages.
- They have a closed circulatory system: heart, arteries and veins.
- The sexes are separate in most vertebrates.
Some examples of vertebrates are: cows, dogs, fish and frogs.
Animals like snails houseflies mosquitoes do not have a backbone. They lack a spine or a vertebral column. These animals are called invertebrates. Thus snail’s houseflies mosquitoes are invertebrates. About 98% of the total animals are invertebrates. These are about 2 million species of invertebrates.
Besides not having backbone invertebrates have the following characteristics:
· They are unicellular or multicellular.
· They don’t have an internal skeleton made of bones. Many invertebrates have a fluid-filled, hydrostatic skeleton such as the jelly fish the worm. Others have a hard outer shell such as insects and crustaceans.
· They lack cell walls.
· They reproduce sexually.
· They are small in size and slow moving.
· They are heterotrophic organisms.
Some examples of invertebrates are: leech, slug, ant, and butterfly
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