ANATOMY AND CLASSIFICATION OP THE ARENICOLID.E. 533 



the presence of thirteen pairs of nephridia and the large 

 genital sacs appended to them (except the first pair) in A. 

 ecaudata is a feature by which this species is sharply dis- 

 tinguished from all the rest. The balance of evidence does 

 not seem suflficient to justify the union of A. Grubii and A. 

 ecaudata into a distinct genus, although they form a distinct 

 section of the genus. 



The genus Arenicola may, then, be divided into two 

 sections, the first containing A. marina, A. Claparedii, A, 

 cristata, characterised by a well-marked prostomium, a well- 

 developed brain with anterior and posterior cerebral lobes, 

 the presence of six pairs of nephridia in segments 4 to 9 

 or five pairs in segments 5 to 9, and the absence of the 

 gills and parapodia from the posterior region. The second 

 section contains A. Grubii and A. ecaudata, with simple 

 conical non-lobate prostomium and commissural brain, and the 

 gills and setigerous sacs continued backwards to the pygidium. 

 Of the first section A. Claparedii is the most distinct. 

 The prostomium is complexly folded, and produced laterally 

 into short hollow processes into which the anterior cerebral 

 lobes are extended. The otocysts are absent. The meta- 

 stomial grooves are very distinct, and expanded at their origin 

 just behind the nuchal groove. This expansion has been 

 previously considered (Ehlers) as indicating the position of 

 otocyst, but there is a similar expansion of the groove in A. 

 marina, and it does not occur in the region accurately corre- 

 sponding to the opening of the otocyst. Internally, the 

 multiple oesophageal glands and the absence of diaphrag- 

 matic pouches are distinctive. Five pairs of nephridia are 

 present. 



A. marina and A. cristata are closely similar to each 

 other in many points, such as the prostomium, brain, and 

 many of the external characters. The former, however, has 

 two additional pairs of gills. The presence of a nephridium in 

 the third chtetigerous segment, opening just behind the fourth 

 row of neuropodial ch^tae, and the possession of an external 

 aperture to the otocyst, and presence of foreign particles 



