Phylum Annelida

1. Annelids occur in freshwater, seawater, or moist soil. Some are free-living, some are burrowing and a few are parasitic.
2. The body is segmented broadly divided into ring-like true segments.
3. The body is soft extensile and triploblastic.
4. Organ level body organization.
5. The first animal to acquire true coelom.
6. Coelomic fluid act as hydraulic skeleton.
7. Both longitudinal and circular muscles are present.
8. Straight and complete alimentary canal.
9. Excretion with the help of nephridia.
10. The first animal to have a close circulatory system.
11. Blood is red due to hemoglobin dissolved in plasma.
12. The mode of respiration is cutaneous respiration through moist skin.
13. Locomotion organs are segmentally arranged paired lateral appendages, parapodia, chitinous setae, or chaetae.
14. Sex may be unisexual (nereis) or maybe bisexual (Earthworm).
15. Fertilization is external or internal.
16. Development is direct or indirect, there is a free-swimming larvae stage(trochophore).
17. The nervous system consists of a dorsal “brain” and a ventral nerve cord, having ganglia and lateral nerves in each body segment.

Animals have definite shapes and sizes. They are unbranched except for the sponges. Animals’ organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc. The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They lack chlorophyll and are heterotrophic in their mode of nutrition. Animals have distinct excretory organs. Asexual reproduction occurs only in the lower animals. Animals are classified according to some basics like the number of germ layers present in the embryo; the symmetry of the body of the organism, mode of origin of mouth. The broad classification of Animalia is based on common fundamental features. The phyla of the animal kingdom are Porifera, Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata. Hereunder is a detailed explanation of the phylum Annelida.