PHYLUM ANNULATA 



431 



noto 



rwuro 



vent.cirr 



FIG. 353. Nereis dumerilii. A single para- 

 podium, magnified : ac. aciculum ; dors. cirr. 

 dorsal cirrus : neuro. neuropodium ; noto. noto- 

 podium ; vent. cirr. ventral cirrus. (After 

 Claparede.) 



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



somewhat longer stout appendages or palpi (palp). The peri- 



stomium, which has some 



resemblance to the seg- 

 ments of the 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 



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 ar<> 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. 353) is examined more attentively it is 

 found to be biramous, or to consist of two dis- 

 tinct divisions a dorsal, which is termed the 

 notopodium (noto), and a ventral, which is called 

 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 invagina- 

 tion of the epidermis the setigerous sac and is 

 capable of being protruded or retracted and 

 turned in various directions by strands of 

 muscular fibres in the interior of the para- 

 podium. 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. 354) are exceedingly fine, but stifiish, 

 chitinous rods, of which two principal kinds are 

 recognisable : both have a terminal blade arti- 

 culating with the main shaft of the seta by a 

 distinct joint ; but in the one variety the shaft 



Fl mer\iiT7 I seto i highiy of the seta is finer than in the other, and the 



Sa l p 8 ar 1 ed ti e) e . d ' ( ^ ieT terminal blade long, slender, and nearly 



straight, whereas in the other variety it is 



short and slightly hooked. On the dorsal side of the parapodium 



is a short cylindrical, tentacle-like appendage, the dorsal cirrus 



(Fig. 353, dors, cirr), and a similar, somewhat shorter appendage, 



