PHYLUM ANNULATA 



471 



Neither prostomium nor peristomium bears parapodia, though an 

 aciculum is sometimes developed in the latter ; the prostomium 

 in fact, is not quite correctly termed a segment, lieing different 

 from the true segments both in structure and in mode of develop- 

 ment. In the Oligochseta there is no definite head, tentacles are 

 entirelj' absent, and in the terrestrial forms the prostomium does 

 not lodge the cerebral ganglion. In Sternaspis spinosa the pro- 

 stomium is elongated and bifurcated like the proboscis of the 

 Gephyrea armaia (vide infra). 



Fia. S70. — Folyjioe extenuata. Dorsal view of anterior extremity, dors. clrr. dorsal cirri ; 

 tl. elytra ; perist. tent, periatomial tentacles ; prmst. prostooiium. (After Clapar6de.) 



The last segment is termed the anal segment, owing to its 

 bearing the anal opening ; it usually also differs from the preceding 

 segments 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 Polychasta. Sometimes they 

 occur on all, or nearly all, the segments ; sometimes they are 

 confined to the middle region of the body ; sometimes they are 

 present only at the anterior end, as in the majority of the Poly- 

 chseta living habitually in tubes (Figs. 366 and 373). In the 



