PHYLUM ANNULATA 



441 



dors, cirr 



noto 



rieuro 



FIG. 348, A. Nereis dumerilii. A single para- 

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

 dorsal cirrus : neuro. neuropodium ; noto. noto- 

 podium ; vent. cirr. ventral cirrus. (After 

 Claparede.) 



on its dorsal surface four large rounded ci/rx, in front a pair of 

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

 what 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 

 (pcrist. tent): on its ventral 

 aspect is a transversely vent.cirr 



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 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. 348, A) 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 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 invagination of the epi- 

 dermis the sctifjerous sac 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. 348, B) are exceedingly 

 fine, but stiffish, chitinous rods, of which 

 two principal kinds are recognisable : both 

 have a terminal Uadc 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 



1 



jo. 348, B. Nereis du- 



merilii. Seta; highly mag- 

 nified. (After Claparede.) 



