ANATOMY OF ARENICOLA ASSIMTLTS. 739 



characters are briefly described. Ehlers states that this 

 species closely resembles A. marina in general external 

 characters, but that in A. assimilis there are twenty 

 chfetigerons segments and as a rule there are thirteen pairs 

 of gills, the first being- situated on the eighth chaBtigerons 

 annukis, but occasionally only twelve pairs of gills are 

 present. Ehlers also finds that, compared with A. marina, 

 the median lobe of the prostomium of A. assimilis is 

 propoi'tionately smaller than the lateral ones, and the noto- 

 podial setfe somewhat more feebly feathered. 



The New Zealand specimens differ from Ehlers' species in 

 the number of cha;tigerous segments and in the position of 

 the first gill, but unfortunately Ehlers does not mention the 

 nephridia or nephridiopores, the oesophageal glands or the 

 otocysts, — important diagnostic characters concerning which 

 information was essential before the affinity of the New 

 Zealand specimens with A. assimilis could be either 

 accepted or rejected. My thanks are due to Dr. Michaelsen, 

 of the Hamburg Museum, who collected the specimens 

 examined by Ehlers, for kindly sending to me two complete 

 examples of A. assimilis from Uschuaia, in Tierra del Fuego, 

 and an incomplete specimen from Punta Arenas, in the Straits 

 of Magellan. 



As Ehlers has given only a brief account even of the 

 external characters of his species,^ I propose to describe the 

 pi'incipal features of the three specimens given to me by Dr. 

 Michaelsen before proceeding to consider the New Zealand 

 specimens in detail. 



^ Since wriling tlie aljove I have received, lliiougli tlie kindness of Pro- 

 fessor Ehlers (1901), a copy of his recently published monograph, 'Die Poly- 

 chseten des magellanischeu und chilenischeii Strandes,' in which he describes 

 (pp. 177, 178) tiie structure of the otocysts of A. assimilis, and also states 

 that tlie alimentary canal, the vascular system, and the nephridia of tills 

 species agree, so far as he can ascertain, with those of A. marina. Tliis 

 agreement is, however, not quite so close as Ehlers' statement would lead 

 one to expect, since, for example, the oesophageal glands in the latter species 

 are a single pair, while in A. assimilis there are several pnirs. These poinis 

 are furtlier discussed below. 



