ANATOMY AND CLASSIFICATION OF THE AKENICOLIDJ]. 535 



which the gills were either not present or not in the full 

 number. 



Hitherto only three species have been seen in this stage, 

 A. marina, A. cristata, and A. ecaudata, and each of 

 these differs from the adult in the arrangement and the form of 

 the gills, setaG, and to a lesser extent the form of prostomium ; 

 but can only be adequately determined by the nature of the 

 otocysts, oesophageal pouches, and nephridia. The gills arise 

 in A. marina as hollow papillte in the centre of the branchial 

 region, and are marked by their efferent and afferent vessels. 

 Their rate of development appears to vary, but possibly is 

 constant for the young of the ''Laminarian " or pinnate-gilled, 

 and "^ littoral " or dendritic-gilled varieties respectively. 

 Hence the gills are not valuable for determining this species. 

 The absence of chtetae from the tail gives a characteristic 

 appearance to that region, but probably A. Claparedii is 

 similar in this respect, though its post-larval stage has not 

 been described. The chsetse themselves differ in A. marina 

 from previous descriptions of the post-larva. Notopodially 

 there are hastate and capillary forms ; neuropodially the 

 crotchets have the rostrum bent very strongly on the axis, 

 and above it there may be one or two (rarely three) teeth ; 

 below it a minute process is often present. We are thus com- 

 pelled to fall back on the internal organs; and the combination 

 of their characters — the open otocyst with foreign otoliths, the 

 single pair of oesophageal pouches, and the small pair of 

 nephridia in the third chastigerous segment unrepresented in 

 any other species at present known — suffices, and is only suffi- 

 cient, to characterise post-larval A. marina. 



Somewhat similar is the case with A. ecaudata. It is 

 probable that the post-larval stage of A. Grubii will soon be 

 known ; indeed, since the latter is the commoner of the two 

 it is singular that its post-larval stage has not already been 

 met with. The post-larvce of either could be differentiated by 

 the sectional character — the extension of neuropodia and 

 notopodia almost to the hinder end of the body, and the 

 otocysts ; from each other by the presence in A. ecaudata of 



