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



serving as otoliths, distinguish A. marina. A. cristata 

 sometimes, but by no means always, has small cirriform pro- 

 cesses on the achsetous caudal region. One diagnostic feature 

 of A. cristata is the closed otocyst with its single large 

 otolith formed by secretion of the walls of the vesicle, and in- 

 creasing in size apparently throughout life. There are six 

 pairs of nephridia and sometimes a seventh, but the first pair 

 correspond to the second nephridia of A. marina. 



Enough has already been said to differentiate the members 

 of the second section. The large number of nephridia and 

 the size of the gonads readily separate A. ecaudata from A. 

 Grubii, which has only the five pairs of nephridia found in 

 some of the first section of the genus. The gills commence 

 on the sixteenth chaetigerous segment in A. ecaudata, on 

 the twelfth in A. Grubii ; and this arrangement is so constant 

 that in rare cases (A. Grubii) where the gills commence on 

 the thirteenth segment, the first branchiferous segment should 

 then be reckoned really as equivalent to the second in the 

 typical arrangement. The arrangement of afferent and effer- 

 ent vessels, and their presence, even when the true first gill is 

 very small (or absent), fully confirm this manner of regarding 

 the position of the first gill as a really fixed point. In this 

 connection we may remark that it would conduce to reliable 

 systematic work if the doubtful species A. branchial is, 

 Audouin and Milne-Edwards, were altogether neglected, since 

 the original description does not accord with that of any 

 accurately known species of Arenicola ; no naturalist has found 

 an undoubted specimen of that form, though many have 

 worked at the locality (St. Vaast) where it was obtained by 

 Audouin and Milne-Edwards. Lastly, the original speci- 

 mens at Paris and those named by Johnston in the British 

 Museum have been lost. 



Having settled the diagnostic features of the species of 

 Arenicola as seen in adult forms, we must now shortly dis- 

 cuss how the post-larval stages can be discriminated, using 

 the term post-larval in the sense already indicated, as imply- 

 ing an Arenicola with the full number of segments, but in 



