Introduction:

The name Annelida is obtained from word anellus, that means little ring and appropriately explains ringed appearance of annelids. Segmentation is key innovation for this group. Annelid worms demonstrate great advancement over lower phyla like Cnidaria, Platyhelminthes and Nematoda. Their most characteristic feature is segmentation of the bodies, both externally and internally (metamerism). In some annelids, different segments exhibit great diversity of structure and function. Segments can be independently controlled for different functions as each segment has its own excretory organs, and nerve concentrations which control actions of segment known as ganglia. Annelida have not only kept basic characteristics introduced in acoelomates and pseudocoelomates like triploblastic body structure, bilateral symmetry, and the body cavity, but have also made huge improvements in certain of these structures also contributed some new ones. Body cavity in annelids is true coelom (enclosed by epithelial cells of mesodermal origin), nervous system is more centralised, and cephalisation further accentuated.

Diagnostic and special characteristics of Annelida:

• Bilaterally symmetrical, vermiform (worm-like).
• Body more than 2 cell layers thick, having tissues and organs.
• The muscular gut with mouth and anus.
• Body separated in segments (segmentation may not be noticeable externally, but is always obvious in nervous system).
• Pre-segmental prostomium having nervous ganglion, and post-segmental pygidium.
• Body cavity the series of schizocoels, obscured in specimens with anterior and posterior suckers.
• Body cavity frequently subdivided by transverse septa, but often suppressed or obscured in some or all segments.
• Outer epithelium enclosed by the cuticle and with epidermal bristles or chaetae in bundles or individually, except in specimens with anterior and posterior suckers (leeches).
• Body wall muscular, frequently with complete circular muscle layers and 4 blocks of longitudinal muscles.
• Segmental ducts of mesodermal and ectodermal origin, that may be combined, limited to one or a few segments and/or partly suppressed.
• Larva may or may not be present; if present it is of trochophore type.
• Planktonic development in marine forms sometimes using free-living trochophore larva but this stage often encapsulated.

Diversity of annelids:

There are approx 12,000 species which comprise sandworms, bristleworms, fanworms,ragworms, earthworms and leeches. The majority of them are marine, but they are also common on land and in freshwater.

Classification of annelids:

There are 3 classes of Annelida named on basis of presence or absence of chaetae and on their number and nature, are given below:

Class Polychaeta - these are marine worms including worms which are usually known as fan worms, feather worms, bristleworms, tube worms, sandworms, ragworms, etc.

Class Oligochaeta - earthworms; both terrestrial and freshwater.

Class Hirudinea - leeches; generally freshwater, some terrestrial and marine.

Class Polychaeta:

The name signifies many bristles, therefore common name, marine Bristle worms. There are more than 5,500 species contained generally marine, and few freshwater and terrestrial species.

The class characteristics of polychaetes are as follows:

• External and internal segmentation.
• Lateral biramous (two-branched) parapodia (fleshy paddlelike appendages) that carry numerous setae/chaetae (or bristles).
• Chaetae are chitinous (comprised of chitin).
• Generally strongly cephalised and specialized with eyes, and head appendages (palps and tentacles).
• Fertilization is external.
• Several live in connection with sponges, echinoderms, molluscs, crustaceans.
• They have no suckers.
• Gonads are localized but extend throughout body.
• Like Nereis, Glycera, Sabella, Aphrodite, Arenicola, Eunice, etc.

The head proper is well-developed and comprised of prostomium and peristomium. Triangular prostomium projects above mouth and bears one pair of tentacles, 1 pair of fleshy palps, and 2 pairs of eyes. Peristomium, surrounding mouth, is first true segment. It bears 4 pairs of peristomial tentacles. Each segment behind the peristomium is similar and each bears a pair of fleshy parapodia laterally.

Class Oligochaeta (Earthworms):  

The word Oligochaeta signifies few bristles. There are more than 3,500 known species. Oligochaetes vary from polychaetes in that most are terrestrial or freshwater. Oligochaeta are said to have possibly evolved from Polychaeta, or at least from some common ancestor. Several of the differences between two classes can be obsereved as adaptations to burrowing terrestrial life shown by oligochaetes. These comprise: reduction of head, absence of parapodia, and greater complexity of reproductive system.

Features of class are given below:

• Segmented externally and internally.
• Absence of parapodia but with setae/chaetae arranged singly, arising from body wall.
• Head generally poorly developed (prostomium simple) and lacking appendages.
• Lack eyes - but have sensory systems which detect light, touch, and moisture.
• With the clitellum (that secretes cocoon) and spacious coelom.
• Most species are terrestrial or freshwater.
• Eat soil and organic debris.
• Castings are deposited at surface.
• Reproduction is by copulation and cross fertilization.
• E.g. Lumbricus terrestris, Tubifex tubifex, etc.

Class Hirudinea (leeches):

There are more than 500 species of marine, freshwater and terrestrial leeches. They are mostly known for their ectoparasitic blood sucking habits, though several leeches are freshwater and marine predators. The anterior and posterior sucker is present and correlate with limitation to small, fixed number of segments. Parapodia and setae are not present and segmentation obscured by development of secondary annuli and loss of septation. Photoreceptors light sensitive cells are typically grouped in distinct eyes. Coelom is decreased to system of sinuses and.

Leeches secrete the anticoagulant termed hirudin, that is injected in wound, as in some leeches; it may be reserved for preventing clotting of blood in crop, as in Hirudo, that also secretes the histamine in wound to cause dilation of blood capillaries of host to improve bleeding.

Features

• They are very specialized annelids.
• Several are ectoparasites of vertebrates, whereas some are free-living predators or scavengers.
• They happen mostly in freshwater with few on land and in sea.
• They lack a distinct head.
• Their body is rather shortened; all members of this class contain 33/34 segments.
• They lack chaetae or parapodia or head appendages.
• They are hermaphrodites.
• Growth is direct within cocoons secreted by clitellum.
• E.g. Hirudo medicinalis etc.

Annelids are triploblastic, bilaterally symmetrical, metamerically segmentated invertebrates, with the true coelom. Except in leeches, coelom is partly subdivided by septa.

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