POLYCHAETA 235 



The diagnostic features of Nereis and other genera mentioned in 

 the classification are given below. 



Nereis (Fig. 173). 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. 174 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. 174 C). Similar to Eulalia but a pelagic polychaet 

 with transparent body and enormous eyes of complicated structure. 



Syllis (Fig. 174 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. 



Myrianida. Similar to Syllis but pharynx long, with a circle 

 of teeth; no ventral cirrus. 



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. 

 176 A), serpulids (Fig. 177) and sabellids, the appendages of the head, 

 which probably correspond to tentacles, are very numerous. Each 

 tentacle has a ciliated groove running from the tip to the mouth and 

 along this minute particles may be seen to travel. In the terebellids 

 these tentacles are extensible and capable of independent movement 

 when separated from the body. In the serpulids and sabellids, they 

 are rather stiflF branched structures, which can, however, curl up 

 when withdrawn into the tube ; they sometimes bear eyes and some- 

 times are wonderfully pigmented. 



Besides the food-collecting tentacles there are gills in the tere- 

 bellids. These are branched processes, usually three pairs, situated 

 just behind the head, full of circulating blood. In the serpulids and 

 sabellids, there are no special respiratory organs but the whole 

 surface of the body serves for the exchange of gases. 



In the terebellids the tubes are composed of a soft cementing 

 substance mixed with mud or a parchment-like material to which 



