460 



ANNELIDA. 



When they are biramous the dorsal branch is called the notopodium r 



and the ventral the neuropodium. 



The acicula are especially strong, dark- 

 coloured setae, hardly projecting at all, and 

 deeply embedded in the parapodia (Fig. 364). 

 Typically there is one in the noto- and one 

 in the neuropodium ; they serve for the 

 attachment of the muscles of the setae. 

 The setae are chitinous, and project in 

 groups from sacs on the parapodia ; each 

 seta is formed by a single large cell at 

 the bottom of the sac. 



The form of the setae varies extremely, 

 and affords a good character for the classifi- 

 cation of families and genera. According 

 to the strength, form, and mode of ending 

 the following forms may be distinguished : 

 simple setae, which may be hair-like or 

 flattened (paleae). or lance-shaped, or curved 

 at the end (crotchets), etc.; jointed setae 



(composite), which carry a terminal articulated appendix (Fig. 365, g) 

 found in the Nereidiformia ; uneini (a, b), setae with a sharply-- 



PIG. 363. Nereis margaritacea 

 (after M. Edwards). Head 

 with everted mouth and pro- 

 truded jaws (A"). The small 

 spines borne by the eversible 

 wall of the mouth-cavity are 

 shown. The prestomiuin 

 carries at its front end two 

 tentacles F, and ventrally 

 two palps P. The peristo- 

 mium carries four tentacular 

 cirri Fc on each side. 



op bm 



FIG. 364. Section through a segment of a Polychaet, diagrammatic (from Lang), ac aciculum ; . 

 1) setae ; bm ventral nerve cord ; dc dorsal cirrus ; dp notopodium ; k gill ; hn longitudinal 

 muscles; md intestine ; np nephridium ; ov ovary; rm circular muscles ; tm transverse muscle ; 

 tr funnel of nephridium ; vc ventral cirrus ; vd dorsal, vo ventral vessel ; vp neuropodium. 

 There are ova in the body-cavity. 



