EXTERNAL FORM HEAD 



259 



In Aphroditidae and certain Amphinomidae the body is more 

 or less oval in shape. In Lipobranchius 

 and Sternaspis it is grub-like, short, and 

 cylindrical, with rounded ends ; in the 

 former it is difficult to distinguish head and 

 tail, or dorsal and ventral surfaces. 



The segments composing the trunk may 

 be all alike, or may constitute two more or 

 less sharply marked regions, the thorax and 

 abdomen, differing in the character of the 

 chaetae, or in their arrangement, or in some 

 other way, as in the Sabelliformia and the 

 Capitelliformia. 



As peculiar cuticular structures, the 

 curious shields of Sternaspis, and of certain 

 of the Maldanidae may be mentioned. 



The posterior extremity is generally more 

 or less narrowed, and most of the Nereidi- 

 formia are provided with special elongated 



cirri, borne by the anal segment. In the FIG. 131. Sabeiiaria al- 

 Maldanidae and others the body terminates 

 in a funnel, at the bottom of which is 

 placed the anus. Only in a few cases is the anus not terminal ; 

 in Notopygos and other Amphinomidae, as well as in some species 

 of Polynoe, it is dorsal. 1 In Sabeiiaria and Pectinaria the hinder 

 end of the body undergoes great degeneration ; in the former it 

 is achaetous, but cylindrical and bent forwards alongside the body 

 (Fig. 131). In Pectinaria (Fig. 177), this region, which is 

 called the " scapha," is leaf-like, and serves to close the narrower 

 end of the tube in which the worm lives. Arenicola marina 

 and some Terebellids have no chaetae in the hinder, narrower 

 part of the body. 



The Head. The prostomium is, in the majority of cases, 

 rounded or conical, though it may be square (Nephthys) or 

 elongated and jointed (Glycera), or even hammer-shaped (Tomop- 

 teris) ; or it may be fused with the peristomium, and apparently 

 absent (Arenicola}. In the great group Cryptocephala, the peri- 

 stomium grows forwards so as to hide the prostomium entirely. 



In a few of the Nereidiformia the prostomium is compressed, 

 1 In Coabanyia (see p. 284) the anus is near the anterior end, on the ventral surface. 



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