TYPE ANNELIDA. 



205 



\ c 



FIG. 



98. PARAPODIUM OP 

 Aferets virens. 



br = bi-Niiclim. 

 dc = ( i ()rsa i ciri . us . 



s = setae. 

 vc : : ventral cirrus. 



cations of the cirri or brandies arising from them, uml 



respiratory in function, also occur. Muscles pass from the 



body-wall to the parapoclia, which thus 



become important organs of locomo- 



tion and in some of the actively swim- 



ming species assume a more or less 



flattened plate-like form. 



The head segment is generally well 

 differentiated from those which succeed 

 it, being destitute of parapodia and 

 setfle, and as a rule carrying a number 

 of appendages sensory in function, and 



being likewise usually provided with 



mi TV i 



eyes. The cephalic appendages may 



* x , . ' 



be short and rather stout, forming 



' 



what are termed palpi (Fig. 100, p), 

 or somewhat longer and more slender, 

 forming the cirri (c), or even still more 

 slender, being then known as tentacles (). 



The body is enclosed in a chitinous covering secreted by 

 the subjacent ectoderm, here known as the hypodermis (Fig. 

 99, liy}. The musculature of the body-wall which lies below 

 the hypodermis is separated from this by a basement-mem- 

 brane and consists of an external layer of circular fibres (cm) 

 and a subjacent layer of longitudinal fibres (Im) which is, as 

 a rule, interrupted in the mid-dorsal and ventral lines and 

 also in the region of the two lobes of the parapodia so as to 

 form four bundles. Special muscles extend from the body- 

 wall to the base of the seta-sacs, and furthermore a pair of 

 muscle-bauds cross the cavity of each metamere, in typical 

 cases passing from the lateral regions of the dorsal surface 

 downwards and inwards to be inserted into the ventral bod}-- 

 wall on each side of the median line. The inner surface of 

 the longitudinal muscle-layers is lined with a layer of peri- 

 toneal cells which completely enclose the coelom (cd) of each 

 metamere, being reflected up t on the surfaces of the dissepi- 

 ments which form the internal partitions between adjacent 

 metameres. The separation of the coalomic cavities of the 

 rnetameres is, however, rarely perfect, openings occurring here 



