PHYLUM ANNUL AT A 



405 



^^ neuro 



vent.clrr 



319. Nereis dumerilii. A single para- 

 podium magnified : ac. aciculum ; dors. cirr. 

 dorsal cirrus ; neuro. neuropodium ; noto. noto- 

 podium ; vent. cirr. ventral cirrus. (After 

 Claparede.) 



former bears on its dorsal surface four large rounded eyes 



in front a pair of short cylindrical tentacles (tent.), and further back 



a pair of somewhat longer 

 stout appendages or palpi 

 (palp). The peristomium, 

 which bears some resem- 

 blance to the segments of the 

 v _. 7ZC ^. body, though wanting the 

 parapodia, bears laterally 

 four pairs of long slender 

 cylindrical tentacles (perist. 

 tent.) : on its ventral aspect 

 is a transversely elongated 

 aperture, the aperture of the 

 mouth. The segments of the 

 body differ little in external 



characters from one another throughout the length of the worm. 



Each bears laterally a pair of parapodia, which in the living animal 



are usually in active movement, aiding in 



creeping, or acting as a series of oars for 



propelling it through the water. When 



one of the parapodia (Fig. 319) is examined 



more attentively it is found to be biramous, 



or to consist of two distinct divisions a 



dorsal, which is termed the notopodium 



(noto), and a ventral, which is termed the 



neuropodium (neuro). Each of these is 



further subdivided into several lobes, and 



each bears a bundle of setae. Each of the 



bundles of setae is lodged in a sac formed 



by invagination of the epidermis, the .s&tigcr- 



ous sa-c, and is capable of being protruded 



or retracted and turned in various directions 



by bundles of muscular fibres in the interior 



of the parapodium. In each bundle there 



is, in addition to the ordinary setae, a stouter, 



straight, dark-coloured seta (ac.), the pointed 



apex of which projects only a short distance 



on the surface ; this is termed the aciculum. 



The ordinary setae (Fig. 320) are exceedingly 



fine, but stiffish, chitinous rods, of which 



two principal kinds are recognisable ; both 



have a terminal Nade articulating with the 



main shaft of the seta by a distinct joint; 



but in the one variety the shaft of the seta 



is finer than in the other, and the terminal blade long, slender. 



and nearly straight, whereas in the other variety it is short arid 



FIG. 320. Nereis dumer- 

 ilii. Meta: highly magni- 

 fied. (After Claparede.) 



