General characters of phylum Annelida:

General characters of phylum Annelida:

1. Habitat. These are mostly aquatic and are either fresh-water (e.g. Hirudinaria) or marine (eg. polychaetes like Nereis, Aphrodite). Some are burrowing (e.g. Aphrodite) or tubicolous (e.g. Chaetopterus) while some are found in the moist soil (e.g. earthworm) or parasitic (e.g. Hirudinaria, Pontobdella).
2. Body segmentation. These are long, cylindrical, soft bodied, vermiform or flattened segmented worm. Their body is externally divided into segments called metameres or somites by ring-like grooves called annuli. Number of body segments is fixed in obligochaetes and hirudines, while polychaete annelids add segments throughout their lives.
3. Symmetry. These are generally bilateral symmetrical. 4. Germ layers. These are triploblastic animals.
4. Organisation. These have organ-system organisation.
5. Body plan. These have tube-within-tube body plan.
6. Body cavity. These are true coelomates and the coelom is schizocoelic in origin. Coelom is filled with a watery coelomic fluid which acts as hydraulic skeleton and helps in locomotion. In leeches, coelom is reduced by botryoidal tissue which is probably excretory in function. Coelom is divided into a series of compartments by transverse septa but in the leeches, there is little or no coelomic space.

8.Metameric segmentation. In this, external division of body by annuli corresponds to internal division of coelom by septa. Most of the segments contain the same set of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent.

9. Body wall. It is formed of outer non-chitinous but collagenous cuticle secreted by underlying single layered epidermis of columnar cells. It contains several types of unicellular gland cells and sensory cells, Musculature is formed of only smooth muscle fibres and are of 2 types: outer circular muscle fibres in layers and inner bundles of longitudinal muscle fibres which give high contraction power to the annelids. Innermost layer of body wall is a mesodermal parietal peritoneum except leeches. Cuticle is never moulted.
10. Body appendages. These are present only in the members of class Polychaeta and are unjointed leg-like, called parapodia.
11. Locomotory organs. Most of annelids have segmentally arranged, chitinous bristles, called setae or chaetae, as their locomotory organs. Setae are either embedded in the epidermal setal sacs of body wall (eg. oligochaetes) or borne on parapodia (eg. polychaetes-para lateral; poda = feet). Leeches have suckers as the locomotory organs. Chaetae are formed of moderately flexible ß-chitin and are formed by the chetoblast located at the end of the follicles. Parapodia are unjointed segmental extensions of the body wall found in most of polychaetes especially in crawling or swimming polychaetes (eg. Nereis) while in tubicolous polychaetes, these are simply as slightly raised ridges.
12. Grt. Lanentary canal is complete, straight tube of varying diameter. It has mouth (for ingestion) at anterior end and anus (for egestion) at posterior end of body, so being tail- less. It is held in position in coelom by dorsal mesenteries (e.g. Nereis) or by transverse septa (e.g. in earthworm) or by botryoidal tissue (eg. in leeches). Anus lies on the under surface of the pygidium. Many polychaetes have a muscular pharynx which can be everted eg., Nereis, while some polychaetes (eg, Chaetopterus) are filter feeders. Digestion is always inter- or extracellular.
13. Respiratory organs. Most of annelids show cutaneous (skin) respiration. Some aquatic annelids show branchial (gill) respiration eg. Amphitrite, Terebella.
14. Circulation. Annelids are the first animals to have blood vascular system. In most of annelids, it is closed type (eg. oligochaetes and most of polychaetes) while in leeches, it is formed of haemocoelomic system in which haemoglobin is dissolved in coelomic fluid running in interconnected haemocoelomic channels. Blood is formed of amoeboid blood corpuscles (resembling the leucocytes) and plasma. Plasma is generally red or green coloured due to dissolved erythrocruorin (red) or chlorocruorin (green).
15. Excretion. Excretory organs in annelids are rarely internally closed protonephridia (eg. a few polychaetes with reduced or no circulatory system) but most of annelids have segmentally-arranged and coiled tubular structures called nephridia. These are micronephric (microscopic) in oligochaetes and macronephric (large sized) in many polychaetes (eg. Nereis) and leeches. Nephridia are excretory as well as osmoregulatory in function. Nephridium is generally of open type, called metanephridium, and is connected with the coelom by a ciliated nephrostome. Nephridia of Nereis and Hirudinaria and integumentary nephridia of Pheretima are ectonephric (as directly open at body surface by nephridiopore) while septal and pharyngeal nephridia of earthworm are enteronephric (discharge their wastes into alimentary canal). Aquatic forms are generally ammonotelic (eg. Nereis, Hirudinaria) while land forms are generally ureotelic. Earthworm is ammonotelic when well fed but is ureotelic when starved.