234 



THE INVERTEBRATA 



circulates the water from which they possibly obtain dissolved 

 oxygen. In other polynoids (e.g. Lepidonotus, which lives under 

 stones but does not burrow) the elytra can have no respiratory 

 function but are probably protective, spreading over the whole or 



Fig. 175. Transverse sections through different types of Polychaeta. 

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

 QfPomatocer OS, thoTSiii.. Original. aZ. alimentary canal ; cw. coecumof mid gut; 

 ch.m. matted notopodial chaetae; cil. ciliated groove; ^.ti. and t>.i). dorsal and 

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

 longitudinal muscles; obl.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. 



greater part of the back (sometimes bits of sand or shell are attached 

 to special papillae). Not all the dorsal cirri are modified to form 

 elytra: typical filiform cirri are placed on alternate segments. 

 Aphrodite has remarkable segmental coeca of the alimentary canal. 



