460 



ANNELIDA. 



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



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 



PIG. 363. Nereis inargaritacea the bottom of the Sac. 



(after M Edwards). Head The form of the getae yaries extremely, 



with everted mouth and pro- * 



traded jaws (K). The small and affords a good character for the classifi- 

 ^^X* <*<* of families and genera. According 

 shown. The prestomium to the strength, form, and mode of ending 



carries at its front end two ,-, f -,-, . f -i -,- i a 



tentacles F, and ventraiiy the following forms may be distinguished : 

 two palps p. The peristo- simple setae, which may be hair-like or 



mium carries four tentacular 



cirri FC on each side. 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 ; uncini (a, b), setae with a sharply- 



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

 1) setae ; bm ventral nerve cord ; dc dorsal cirrus ; dp notopodium ; k gill ; Im 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. 



