ANATOMY AND CLASSIFICATION OF THE AUENICOLID^. 539 



This seta belongs to tlie segment following the peristomium. 

 It is tempting to regard (with Ehlers) the otocysts as a pair 

 of modified dorsal cirri belonging to the peristomium. The 

 constant position of these organs, the nerve supply from the 

 same part of the oesophageal commissures, establish the 

 homology of the otocysts, but their absence in A. Claparedii 

 and Branchiomaldane, and the absolute lack of knowledge 

 of their mode of development, render it impossible to prove 

 this morphological explanation of them as modified dorsal 

 peristomial cirri. 



The parapodia are but slightly indicated by a transverse 

 neuropodial thickening with vertical row of crotchets, and 

 conical notopodia with capillary set^e. The gills do not 

 appear to be modified cirri, since in development they are not 

 preceded by sensory structures, but arise at once as re- 

 spiratory organs. 



Internally there are two or more pairs of oesophageal 

 pouches. The heart contains a "cardiac body" of partly 

 glandular, partly muscular nature, except in A. Claparedii 

 and A. cristata, where it is absent. Complete septa occur 

 in the tail region of the caudate forms, and through 

 the greater part of the branchial region in the ecaudate 

 species. 



There are constantly three strong septa or " diaphragms " 

 at the beginning of the first, third, and fourth cheetigerous 

 segments. Five, six, and thirteen pairs of nephridia occur in 

 different species, but altogether they occupy fourteen seg- 

 ments (4 — 17). The development is only known in A. 

 cristata. In Arenicola generally, the gonads develop on 

 the peritoneal covering of the gonidial vessel, and pass out 

 by the nephridia themselves. The segmentation of the egg 

 in A. cristata is complete but somewhat unequal. The 

 larva is telotrochous, with a ventral band of cilia and with a 

 pair of eye-spots, and swims freely for a day or so ; afterwards 

 forming a gelatinous case, and developing its segments and 

 other adult organs. Hence the metamorphosis is very 

 slightly marked so far as is known. The various descrip- 



