Glass 1. ChcKtopoda. 169 



body- wall {e.g. in Earthworms). Very rarely the parapodia are 

 present without chsetas; in some forms they are entirely absent 

 from certain segments. There is often a dorsal and a ventral 

 cirrus, tentacle-like appendages, arising from the upper side of the 

 dorsal parapodium (notopodium), and the under side of the ventral 

 parapodium (neuropodium) respectively. In some forms, the dorsal 

 cirri on some or all of the segments are large, and form lamellae or 

 elytra, covering the upper surface of the animal. 



The two anterior segments differ from the others. The first seg- 

 ment, the prostomium, which overhangs the mouth, has no 

 parapodia, but a number (usually 1 — 9, in many tubicolous forms a 

 much larger number, in others none at all] of thread-like appendages, 

 the so-called palpi and tentacles. The second segment, the 

 peristomium, which usually bears the mouth, although sometimes 

 this is still further back, is provided with a rudimentary parapodium 

 on each side, carrying few chsetee or none, but one or two well- 

 developed, f orwardly directed cirri, the so-called tentacular 

 cirri. One or more of the ordinary segments may be fused with 

 the peristomium, which, like the prostomium, may be destitute of 

 appendages ; in this case, their parapodia and cirri more or less 

 resemble the oral appendages. Frequently the oral and following 

 somites are fused and are diflBcult to distinguish. The terminal seg- 

 ment is without chastsB, and is often furnished with two long processes, 

 the anal-cirri. 



The integument is covered with a thin continuous cuticle, 

 but, in spite of this, is often ciliated over certain limited tracts. The 

 skin, with the underlying muscle-layer, forms a strong body-wall, 

 which encloses a spacious body-cavity, very often divided into a series 

 of compartments by transverse septa, corresponding to the con- 

 strictions between the segments. These septa, which are naturally 

 traversed by the alimentary canal, and the blood vessels, are also 

 perforated by holes to allow of the passage of the coelomic fluid. 

 Sometimes the septa are replaced by strands, which pass from the 

 body-wall to the alimentary canal. 



The anterior region of the digestive tract is usually a muscular 

 pharynx, which may be everted like a proboscis, it is frequently 

 provided with chitinous teeth, or hooks, in larger or smaller 

 numbers. The rest of the alimentary canal is generally a straight tube, 

 with constrictions at places where it is encroached on by the body- 

 wall, more rarely it is coiled ; in some short forms (Sea-mouse) the 

 gut is furnished with a double row of caeca. The anus is, as a rule, 

 situated at the posterior end of the body. 



The eyes, which are, however, absent from many Cheetopods, 

 belong to the type figured in Fig. 20, 5—6. They usually number from 

 two to four, and are situated on the prostomium ; in certain Tubicolee, 

 however, on its thread-like appendages ; in a few other forms, on 



