PHYLUM ANNULATA 



433 



aciculum 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 

 structure and in mode of develop- 

 ment. In the Oligochaeta there 

 is no definite head, tentacles are 

 entirely absent, and in the terres- 

 trial forms the prostomium does 

 not lodge the cerebral ganglion. 

 In Sternaspis spinosa the pro- 

 stomium is elongated and bifur- 

 cated like the proboscis of the 

 Gephyrea armata (vide infra). 



The last segment is termed 

 the anal segment, owing to its 

 bearing the anal opening; it 

 differs from the preceding seg- 

 ments also, usually, 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 

 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 Tubicola 

 (Figs. 337 and 342). In the 

 Terebellidw the branchiaa are 

 situated on the dorsal surfaces 

 of some of the anterior segments. 

 In the Serpulidce they form two 

 incomplete lateral circlets of 

 elongated appendages, situated 

 at the anterior end of the body, 

 and apparently appendages of 

 the prostomial segment, sup- 

 ported sometimes by a cartila- 

 ginous skeleton; one of them 

 is enlarged to form a stopper, or 

 opercuhwn (op.), often armed with 

 calcareous plates and spines, 



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