CII.ETOPODA. 



369 



surface [Aphrodite). The appendages of the parapodia present 

 an equally great variety of form and not unfrequently vary in the 

 different parts of the body. They are either simple or ringed tenta- 

 cle-like processes, the cirri, which are distinguishable into dorsal and 

 ventral cirri. The cirri are for the most part filiform, and sometimes 

 jointed or conical, and then are often provided with a special basal 

 joint. In some cases the dorsal cirri are flattened out as broad scales 

 and leaves, the elytra, which constitute a protective covering (Aphro- 

 dite) (fig. 299). In addition to the 

 cirri, branchiae which may be filiform 

 or branched and antler-like, comb- 

 shaped or in the form of tufts, are 

 frequently present ; sometimes they 

 are confined to the middle region of 

 the body, or are extended over almost 

 the whole dorsal surface ; sometimes 

 they are confined to the head or to the 

 anterior segments immediately following 

 the oral segment (cephalic branchiae). 



The two anterior segments may be 

 regarded as forming the head ; they 

 are fused together, and are, with regard 

 to their appendages, different from the 

 following segments (fig. 245). The 

 anterior segment projects beyond the 

 mouth as the frontal lobe, and bears 

 the tentacles and palps [palps are ten- 

 tacular structures arising from the 

 ventro-lateral sides of the head, vide 

 p. 379] and also the eyes ; the posterior 

 cephalic segment or oral segment bears 

 the tentacular cirri. The last segment 

 (anal segment) bears the anal cirri. 



The alimentary canal is usually 

 straight, and extends from the mouth 

 to the anus, which is terminal and rarely dorsal ; it is divided 

 into oesophagus, intestine, and rectum (fig. 300). There is in most 

 cases a dilated muscular pharyngeal bulb which is armed with 

 papillae or with movable teeth and can be protruded as a proboscis. 

 The intestine usually preserves the same structure in its entire 

 length and is divided by regular constrictions into a number of 



24 



Fio. 300. Alimentary canal of 

 Aphrodite aculeata (after M. Ed- 

 wards). Ph, pharynx ; Z>, intes- 

 tine ; L, hepatic appendages. 



