POLYCHAETA 233 



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

 zation carried on much further in the arthropods and vertebrates. 



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

 Errantia or free-swimming forms, but a great number of them (e.g. the 

 Nereids) do live in tubes which, however, they can leave and recon- 

 struct anew. The most beautiful example of tube building in the Poly- 

 chaeta is furnished by Panthalis, a polynoid. In this the chaetal pits of 

 the notopodium produce not stiff bristles but plastic threads which 

 are woven by the comb-like ventral chaetae into a continuous fabric 



m^,". 



'ph.sh. 



Fig. 174. 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; 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. 



which forms the lining of the mud-covered tube. Aphrodite, the sea 

 mouse (Fig. 175 A), is a short, broad form which burrows in mud, 

 and though it does not form a separate tube it covers its back with a 

 blanket made from interwoven chaetal threads similarly formed from 

 the notopodium. Between this blanket and the back is a space into 

 which water is drawn by a pumping action of the dorsal bo^y wall, 

 being filtered through the matted chaetae. In this there are special 

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



