268 THE INVERTEBRATA 



does not form a separate tube it covers its back with a blanket made 

 from interwoven chaetal threads similarly formed from the noto- 

 podium. Between this blanket and the back is a space into which 

 water is drawn by a pumping action of the dorsal body wall, being 

 filtered through the matted chaetae. In this there are special plate- 

 like modifications of the dorsal cirri — the elytra — round which 

 circulates the water from which they possibly obtain dissolved 

 oxygen. In other polynoids (e.g. Lepidonotus^ which lives under 

 stones but does not burrow) the elytra can have no respiratory 

 function but are probably protective, spreading over the whole or 

 greater part of the back (sometimes bits of sand or shell are attached 

 to special papillae). Not all the dorsal cirri are modified to form 

 elytra: typical filiform cirri are placed on alternate segments. 

 Aphrodite has remarkable segmental coeca of the alimentary canal 

 in which takes place digestion of the fine food particles which pass a 

 sieve at the junction with the intestine. 



The diagnostic features of Nereis and other genera mentioned in 

 the classification are given below. 



Nereis (Fig. 184). Two tentacles, two palps; pharynx with two jaws 

 and twelve groups of paragnaths; noto- and neuropodium each 

 double; chaetae all compound; most species have a special sexual 

 form (Heteronereis). 



Eunice. Five tentacles, two palps; pharyngeal armature well 

 developed; a single peristomial segment; gills in many segments; 

 chaetae simple and compound. 



Eulalia (Fig. 185 B). Five tentacles, no palps; pharynx very long 

 with soft papillae only ; three peristomial segments ; dorsal and ventral 

 cirri leaf-like; chaetae all compound. 



Asterope (Fig. 185 C). Similar to Eulalia but a pelagic polychaet 

 with transparent body and enormous eyes of complicated structure. 



Syllis (Fig. 185 A). Three tentacles, two fused palps; pharynx 

 enclosed in a pharynx sheath with a single conical tooth and a mus- 

 cular proventriculus which functions as a pump; no notopodium. 



Autolytus (Fig. 194 B). Like Syllis but pharynx long, with a circle 

 of teeth; no ventral cirrus. Myrianida has similar characters. 



The true tubicolous Polychaeta 

 Here the prostomium has become much smaller and its appendages 

 enormously modified and increased. The peristomium may be pro- 

 duced into a collar which in some forms grows round the prostomium 

 and encloses a funnel-like cavity at the bottom of which lies the 

 mouth. The food consists of small animals or plants or organic debris 

 and it is collected by ciliary mechanisms. In the terebellids (Fig. 

 187 A), serpulids (Fig. 188) and sabeHids, the appendages of the head, 



