PHYLUM ANNULATA 



461 



mouth, and from it also arise the 

 peristomial tentacles. A ventral 

 pair of prostomial tentacles, some- 

 what thicker than the rest, are 

 sometimes to be distinguished, 

 and are termed the palpi. Neither 

 prostomium nor peristomium 

 bears parapodia, though an aci- 

 culum is sometimes developed in 

 the latter ; the prostomium, in 

 fact, is not quite correctly termed 

 a segment, being different from 

 the true segments both in struc- 

 ture and in mode of development. 

 In the Oligochseta there is no 

 definite head, tentacles are en- 

 tirely absent, and in the terres- 

 trial forms the prostomium does 

 not lodge the cerebral ganglion. 

 In Sternaspis spinosa the prosto- 

 mium is elongated and bifurcated 

 like the proboscis of the Echiurida 

 (vide infra). 



The last segment is termed the 

 anal segment, owing to its bearing 

 the anal opening ; it usually also 

 differs from the preceding seg- 

 ments in wanting the parapodia 

 and in having a pair of special 

 cirri, the anal cirri. 



Branchiae are borne on the 

 dorsal surfaces of more or fewer 

 of the segments in many of the 

 Polychaeta. Sometimes they occur 

 on all, or nearly all, the seg- 

 ments ; sometimes they are con- 

 fined to the middle region of the 

 body ; sometimes they are present 

 only at the anterior end, as in 

 the majority of the Polychaeta 

 living habitually in tubes (Figs. 

 372 and 379). In the Terebellidce 

 (Fig. 379) the branchiae are situ- 

 ated on the dorsal surfaces of 

 some of the anterior segments. 

 In the SerpulidcB (Fig. 372) they 

 form two incomplete lateral cir- 

 clets of elongated appendages 



