462 



ANNELIDA. 



or are extended almost across the whole dorsal surface (Euphrosyne) -. 

 sometimes they are confined to the anterior segments immediately 

 following the oral segment (Terebelld)* Both cirri, or one of them, 

 may be altogether absent. 



The last or anal segment is always modified : the parapodia and 

 setae are absent, but one or two pairs of cirri the anal cirri are 

 present. The anus is usually terminal; in Notopygos it is dorsal, 

 and in Caobangia it is anterior and ventral. 



The alimentary canal is usually straight (coiled in Pectinaria, 

 Sternaspis, SipTionostoma, etc.) ; it is divided into buccal cavity, 

 pharynx, oesophagus, and intestine. The buccal cavity has muscular 

 walls, and is often eversible, while the pharynx which also has 

 muscular walls is often protrusible (Figs. 363 and 367). The buccal 

 cavity has a chitinous lining, which may be thickened at certain 



m, 



FIG. 367. Diagrammatic representation of the pharyngeal apparatus of a predatory Chaetopod. 

 g brain; fc jaw; m mouth ; ph pharynx; pt protractor muscles ; rt, ct retractors ; vt wall of 

 buccal cavity (eversible) with papillae or denticles p. In A the apparatus is retracted, in B 

 protruded. (After Lang.) 



spots into denticles, and the pharynx is often armed with powerful 

 and movable chitinous jaws or plates. These structures, however, 

 are not always present (e.g. Terebellids and Sabellids) ; they are well 

 developed in the Nereidiformia. The oesophagus is often provided 

 with a pair of diverticula, which in some Syllidae and Hesionidae 

 contain air. The intestine reaches to the anus, and is constricted 

 segmentally. In the Apliroditidae the sacculations are elongated 

 into caeca (Fig. 368). In Syllids and some Terebellids there is a 

 muscular gizzard with a hard chitinous lining. In the Capitellidae 

 there is a siphonal tube leaving the intestine in front, and entering it 

 again behind. 



The nervous system is constructed on the typical Annelidan plan. 

 It is sometimes in contact with the epidermis, though more often 

 separated from the skin. The cerebral ganglion is contained in 



* The cephalic branchiae of the Sabelliformia are palps not cirri. 



