266 



THE INVERTEBRATA 



From the conical noto- and neuropodia spring a bundle of chaetae ; 

 the chaetal sacs project into the coelom and each bundle is supported 

 by an enlarged and wholly internal chaeta — the aciculum, which also 

 forms the point of origin of the parapodial muscles. The chaeta 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 



Fig. 185. Errant Polychaeta. Peristomial segments stippled to show extent of 

 cephalization. Anterior end. A, Syllis, single peristomial segment; pharynx 

 retracted in sheath, ap. aperture of pharynx sheath cavity ; M. mouth ; p. palp ; 

 ph. pharynx ; ph.sh. cavity of pharynx sheath ; pro. proventriculus ; t. tooth ; 

 ten. tentacle. B, Eulalia, three peristomial segments and five pairs of ten- 

 tacular cirri, pharynx protruded, covered with papillae. B , Parapodium with 

 leaf-like dorsal and ventral cirri, notopodium only represented by dorsal 

 cirrus neuropodium with compound chaetae. C, Asterope, head with five 

 tentacles and three pairs of tentacular cirri (ten.c.) ; conditions in the head 

 region largely governed by the presence of the enormous eyes. Pharynx 

 protruded. 



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

 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. 185 A) this is constituted by a single segment, but 

 usually two or more have been pressed forward towards the mouth 

 and modified. This is the first indication of the process of cephali- 

 zation carried much further in the arthropods and vertebrates. 

 The worms in this group used to be definitely classed as the 



