ANNELIDA. The Phylum Annelida includes the segmented worms. They differ from the Arthropoda in the possession of a perivisceral cavity which is a part of the coelom and from the Mollusca in the fact that the body is segmented. This segmenta tion has its origin in the mesoblastic somites of the embryo, just as the perivisceral coelom is derived from the paired cavities of the embryonic mesoblast. Except for the majority of the Hiru dinea, a few Archiannelida and Oligochaeta, the whole phylum possesses segmentally arranged chitinous chaetae or bristles, from which the name Chaetopoda or bristle-footed worms is derived.
They are with few exceptions elongated, bilaterally symmetrical animals with a body composed of a series of similar segments, not modified to form different parts of the body as in the Ar thropods, from which the Annelids differ also in the unjointed character of their locomotory appendages. A head-region is however normally distinguished, and the thorax is often marked off from the abdomen.
The chaetae (setae) or bristles which are, except for the Brachiopoda (q.v.), specific to this group, are delicate chitinous structures showing immense variability in size and shape.
In its simplest form the coelom consists of a series of chambers which correspond to the segments of the body and are separated from one another by intersegmental septa extend ing from the body-wall to the gut. This condition may be modi fied either in the direction of the suppression of the septa as in some Polychaeta, or of the further sub-division of the coelom as in the more modified Oligochaeta. The latter tendency is most marked in the Hirudinea in which the coelom is partially sup pressed. The coelom communicates with the exterior by a num ber of ways; by the dorsal pores of the Oligochaeta, by the nephridia and by the coelomoducts.
A nephridium is typically a ciliated tube from the coelom to the exterior, consisting of a chain of cells in which a continuous lumen is pierced, and ending internally in a ciliated funnel. Its function is to remove excretory products from the coelom. The coelomoducts, which include the majority of the genital ducts of the Annelids, are somewhat similar structures but the lumen of the tube is intercellular. Moreover according to E. S. Goodrich the true nephridium is an ingrowth from the ectoderm, whereas the coelomoduct is a coelomic out growth. These distinctions are only valid within limits.
This system in the Annelids is typi cally closed, that is to say there is in addition to the coelom a further system of closed spaces, the haemal system, between the gut and the body-wall. A dorsal and a ventral vessel joined by a number of connectives are usually present.
The nervous system consists of a ventral ganglionated cord joining the cerebral ganglia or brain by a cir cumoesophageal commissure. The cord normally lies in the coelom but in a few forms it is embedded in the epidermis.
The gonads are segmental proliferations of the coelomic epithelium.
The Phylum Annelida is usually divided into the following classes :—The Archiannelida, the Polychaeta or marine bristle-worms, the Oligochaeta or aquatic and terrestrial bristle-worms, the Myzostomaria, and the Hirudinea or leeches.
If we except the Echiuroidea, the Chaetopoda are co-extensive with the Annelids.
