232 



THE INVERTEBRATA 



forms the point of origin of the parapodiai muscles. The chaetae are 

 of two kinds, simple and compound. 



The segment (or segments) just behind the mouth, forming the 

 peristomium, is, however, much modified. There are no notopodia or 

 neuropodia (except in occasional species, which retain chaeta-bearing 

 processes as a primitive feature). But the cirri remain as the peri- 



tenx.^. 



ac^' 



neur. 



Fig. 173. Nereis. A, Dorsal view of head and first trunk segments with 

 everted pharynx. B, Side view of same with pharynx retracted. C, Para- 

 podium of unmodified type. D, Parapodium of Heteronereis. E, Example of 

 unmodified compound chaeta. F, Oar-shaped compound chaeta of Hetero- 

 nereis. The peristomium is stippled, pr. prostomium; ten. tentacle; p. palp; 

 ten,c. peristomial cirri ; d.c. dorsal cirrus ; v.c. ventral cirrus ; not. notopodium ; 

 neur, neuropodium ; /./. foliaceous outgrowths of parapodia ; ac. aciculum ; 

 ch. chaetae; ch.' oar-shaped chaetae ;y. jaws; pg. paragnaths. 



stomial cirri in pairs consisting of a dorsal and ventral member. In 

 Nereis there are two pairs of peristomial cirri on each side, indicating 

 the fusion of two segments to form the peristomium. In some families 

 (Syllidae) (Fig. 174 A) this is constituted by a single segment, but 

 usually two or more have been pressed forward towards the mouth 



