ANATOMY AND CLASSIFICATION OF THE ARENlCOLIDi^. 531 



species. For example^ the neuropodial chgetee of post-larval 

 stages of A. ecaudata are totally different from the crotchets 

 of a full-grown specimen, but after examining a long series 

 of preparations we cannot find any constant feature by 

 which the crotchets of full-grown A. ecaudata may be 

 distinguished from those of full-grown A. Grubii. There 

 are differences between adult individuals of A. marina, and 

 there is a considerable amount of difference between the 

 early post-larval setee and those of the full-grown worm. 

 In the post-larva there are two kinds of notopodial bristles 

 (spear-shaped and capillary), and three kinds of neuropodial 

 ones (one-, two-, and three-hooked j, and among the latter the 

 subrostral processes may be distinct, faint, or absent. Again, 

 as Mesnil has pointed out_, sette identical in form with those 

 of neui'opodia occur in the notopodia of early post-larval A. 

 ecaudata, disappearing later on, and the same holds true 

 for A. cristata. When we further consider that the differ- 

 ences between the setse of any of the species is not greater 

 than the difference between the earlier and later formed setee 

 in one individual, the systematic value of these organs 

 becomes in this genus practically nil, although by contrast 

 with other genera they may have a certain generic value, 

 though this is lessened by the distinctly Maldanid character 

 of the setse in the post-larval stages (the curved rostrum and 

 the subrostral process of the neuropodial crotchets, and the 

 hastate notopodial setee) . 



The prostomium has not usually been employed for dis- 

 criminating species, since it is often concealed in the nuchal 

 groove. Moreover anyone who has examined a long series 

 of A. ecaudata and A. Grubii will realise how apparently 

 identical are the prostomial outlines and indentations. But 

 we are able to show that A. cristata and A. marina have 

 quite as similar prostomia, and further that in A. Claparedii 

 there is considerable range of variation, though none of the 

 terms approach to A. marina or A. Grubii. In this respect, 

 however, A. Claparedii is well separated by its compara- 

 tively large and well-developed prostomium. 



VOL. 43, PART 3, — NEW SERIES. 



