436 F. W. GAMBLE AND J. H. ASHWORTH. 



regioninthe "marina" section of the genus cannot, however, 

 be determined by this criterion, since the ch^tigerous annuli 

 are missing. 



The division of the body of Capitellids into thorax and 

 abdomen by the use of regional characters afforded by the 

 sette alone, finds but a feeble analogy in Arenicola, The 

 anterior segments bear as a rule both the capillary setse de- 

 fining the " thorax," and the crotchets which are confined to 

 the "abdomen" of at least some genera of Capitellidae. In 

 Arenicola cristata alone is there an approach to the 

 Capitellid arrangement. For in the first two and in some cases 

 three segments the crotchets are absent or invisible in a 

 surface view. The regional external characters are afforded 

 by the gills, which, as we have already mentioned, are pecu- 

 liar to the genus and the family. 



II. Prostomium. — Though necessarily fused at its pos- 

 terior margin with the peristomium, the prostomium is distinct, 

 and at least in A. Claparedii fairly well developed (PI. 27, 

 figs. 59, 60). It is often stated or implied that this fusion 

 is more extensive, and entails the more or less complete 

 suppression of the prostomium, in Arenicola. Such, however, 

 is not the case; and even in the " e can data " section, where 

 it attains its least development, this structure has fully pre- 

 served its individuality (PI. 22, fig. 5). The misleading state- 

 ments to the contrary are probably due to the retraction of 

 this structure within the nuchal groove, which results from 

 most of the ordinary killing methods. A powerful retraction 

 of the buccal muscles in the living animal also tends to hide 

 the prostomium. 



As is the case with so many other structural features, the 

 prostomium exhibits two types characteristic respectively of 

 the "marina" and the "ecaudata" sections of the genus. 

 In the former it is a hollow, lobate, highly vascular and thin- 

 walled structure ; in the latter it forms a smooth, conical, 

 opaque prominence. The forms which it assumes are more 

 readily understood from PL 22, figs. 2, 5, and PL 27, figs. 59 

 — 61, than from lengthy descriptions. 



