Ecaudate Species of Arenicola 85 



annuli. Gills are not present on the first eleven or fifteen segments ; 

 the main gill-stems, which seem to be of the nature of sympodia, bear 

 branches unilaterally. The prostomium x is simple and non-lobate, it 

 is a transverse band or ridge, generally exhibiting a slight median 

 elevation ; the prostomium merges into the " upper lip " of the 

 peristomium, and forms with this a conical structure overhanging 

 the mouth. Each statocyst is a closed sac, which contains, in the 

 adult, 2 numerous spherical secreted statoliths. The dorsal lip of 

 each nephridium bears on its margin several digitiform, often bifid 

 or trifid, vascular processes; the ventral lip is deeply notched in 

 the middle. The eggs are oval and have a thick vitelline mem- 

 brane (5-6 /*). 



KEY TO THE ECAUDATE SPECIES. 



First gill on the sixteenth segment ; thirteen pairs of 

 nephridia opening on the fifth to the seventeenth 

 segments; gonads large ecaudata, p. 132. 



First gill on the twelfth segment ; five pairs of nephridia 

 opening on the fifth to the ninth segments ; gonads 



small branchialis, p. 138 



(= grubii). 



About sixteen other species of Arenicola have been founded, but, 

 as will be shown subsequently, they must all be merged with one or 

 other of the eight species mentioned in the keys. Attempts have 

 been made previously, but on insufficient data, to merge some of the 

 species; for instance, Ives (1891) believed that all specimens of 

 Arenicola then known could be properly referred to three species, 

 namely, marina, cristata, ecaudata. That suggestion was based on 

 imperfect acquaintance with the external, but especially with the 

 internal, characters of several of the species in question. At that 

 time, and for a few years afterwards, the internal anatomy of all 

 the species, except A. marina, was practically unknown, and that 

 of two of the species has become known only during the last 

 two years, owing to the investigations carried out by the writer in 

 preparation for this Catalogue. 



1 Prof. Fauvel describes the prostomium of A. ecaudata as an obtuse cone, 

 and that of A. branchialis as reduced to a simple transverse ridge. The writer 

 has examined a large number of specimens of both species, but cannot find any 

 constant difference between them in regard to their prostomia ; the variations 

 of form exhibited by the anterior end are due apparently to differences in the 

 amount of contraction taking place on fixation. 



2 In young post-larval stages only a single statolith is present, others are 

 formed subsequently ; but the original one is distinguished from the rest by its 

 greater size. 



