GENERAL SURVEY OF THE CLASS CH&TOPODA. 231 



which functions as an internal skeleton, both by giving support and by 

 serving as an attachment for muscles. With the notopodium, further, 

 true gills containing prolongations of the body* cavity are often associ- 

 ated. Such typical parapodia occur especially in the active free-living 

 forms like Nereis and its allies, but in the order in general the parapodia 

 show much variation, and may be almost suppressed, as in Arenicola. 

 Parapodia are absent from the " prostomium,'' and are rarely fully 

 developed on the first true segment or peristomium. In both cases, 

 however, tactile cirri and tentacles are often present. The prostomium 

 varies greatly in development and structure, and is of great systematic 

 importance ; it is frequently furnished with eyes and other sense organs, 



FIG. 120. Parapodium of " Heteronereis " of 

 Nereis pelagica. After Ehlers. 



i, 2, 3, 4, the leaf-like outgrowths ; A, notopodial cirrus; A, 

 neuropodial cirrus ; a 1 ., a 2 ., acicula or supporting bristles 

 of notopodium and neuropodium ; f., setae. 



but these may also occur in other regions of the body. Apart from the 

 parapodia, the shape and appearance of the body are most affected by 

 the condition of the septa. In the active free-living forms (Errantia) 

 these are usually present throughout the body, and gave a characteristic 

 worm-like appearance. In burrowing and tubicolous forms (Sedentaria) 

 the septa tend to be suppressed. Their absence facilitates burrowing, 

 by permitting free movement of the coelomic fluid, and is often associ- 

 ated with a division of the body into regions, and a loss of the typical 

 uniform shape (cf. Arenicola]. 



With regard to internal organs, the gut is frequently branched and of 

 large calibre. In some cases (Capitellidae) it possesses an accessory 



