POLYCHAETA 267 



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 

 reconstruct anew. The most beautiful example of tube building in the 

 Polychaeta is furnished by Panthalis^ a polynoid. In this the chaetal 



ch.m.x 



,m.d.v. 



nep. 



n.c. C 



Fig. 186. Transverse sections through different types of Polychaeta. 

 A, Aphrodite. After Fordham. B, Arenicola, middle region. After Ashworth. 

 C, Pomatoceros, thorax. Original, al. alimentary canal ; cm. coecum of mid gut ; 

 ch.m. matted notopodial chaetae ; cil. ciliated groove ; d.v. and v.v. dorsal and 

 ventral blood vessels; el. elytron; m.c, m.d.v., m.l. circular, dorsoventral and 

 longitudinal muscles; ohl.m. oblique muscles; nep. nephridium; neur. neuro- 

 podium ; not. notopodium ; n.c. nerve cord ; sin. sinus ; th.m. thoracic mem- 

 brane; v.cir. ventral cirrus. 



pits of the notopodium produce not stiff bristles but plastic threads 

 which are woven by the comb-like ventral chaetae and the shuttle-Hke 

 action of the anterior parapodia into a continuous fabric which forms 

 the lining of the mud-covered tube. Aphrodite^ the sea mouse (Fig. 

 186 A), is a short, broad form which burrows in mud, and though it 



