82 A renicolidae 



The general appearance of these worms is similar to that of post- 

 larval A. ecaudala, but they have about twenty segments fewer. 

 The chaetae are practically identical with those of A. ecaudata , and, 

 as in post-larvae of that species, a crotchet is present in four or five 

 of the last formed notopodia. 



Each statocyst contains one large and about eight minute 

 statoliths. 



The nephridia, which have simple funnels, are short wide tubes, 

 opening posteriorly on the fifth to the ninth segments. Of great 

 interest is the presence, in one specimen, of paired vestigial funnels 

 in the tenth, eleventh and twelfth segments, suggesting that the 

 series of nephridia probably at one time extended further back 

 than it does now. It is possible that these funnels might have 

 disappeared during the subsequent phases of growth, but they are 

 so well marked, that it seems more probable that the three segments 

 which contain them would have been provided with small funnels, 

 similar to that figured by the writer 1 in the tenth segment of an 

 adult specimen of this species. 



Post-larval stages of A. branchialis differ from those of A. ecaudata 

 in having fewer segments and nephridia. 



Examples of Branchiomaldanc vincenti present a general external 

 similarity to post-larval stages of A. ecaudata and branchialis; but 

 the former may be recognised by the bi-annulate gill segments, in 

 which the chaetae and gills are on consecutive rings (cf. Figs. 33, 35, 

 PI. XI), by the presence of gonads and only two pairs of nephridia, 

 and by the absence of statocysts and pigment. 



Separation of the Genus ARENICOLA into Sections and Species. 



The genus Arenicola is divisible into two sections, one the 

 caudate section contains those species, six in number, which 

 possess a posterior region or " tail " upon which neither parapodia 

 nor gills are borne : the other the ecaudate section comprises two 

 species in which the parapodia, and generally also the gills, extend 

 to the posterior end of the worm. In addition to this obvious dis- 

 tinction, the two sections of the genus exhibit several other 

 differential characters, both external and internal, 



1 Q. J. Micr. Sci., xliii (1900), pi. xxvi, fig. 54, 



