54 CHAETODERIMA HAWAIIENSIS. 



Owing to the deliris encrusting; the posterior end of the body it is impossible 

 to determine the position of the dorsal sensory groove in entire specimens. In 

 sections it is seen to occu])y the usual position, that is from the extreme hinder 

 end of the animal forward to a point almost immediately above the level of the 

 anal opening. As is represented Plate 6, fig. 8, it consists of a relatively deep 

 fold of the hypodermis, that anteriorly rapidly disappears but is continuous with 

 a ridge-like elevation in the mid dorsal line extending for a short distance more 

 anteriorly. Some of the spines of the immediate neighborhood are of compara- 

 tively small size and overarch the depression, which is also covered by a thin 

 continuation of the cuticle investing the body. 



The cells of this organ consist of those common to the hypodermis, and others 

 which are much more slender and compact with spindle-shaped nuclei usually 

 subcentrally placed. The latter elements are probably sensory and connect 

 with small groups of ganglion cells holding positions immediately beneath the 

 circular somatic muscles in the neighborhood of the organ, and on the other hand 

 are undoubtedly related with nerves from the branchial ganglia. That this is a 

 definite sense organ and th(^ homologue of the dorsal organ of the Neomeniidae, 

 as maintained by various authors, there is little doul)t, l)ut there is nothing that 

 more definitely establishes its function. 



The gonad extends from the front end of the metathorax to the pericardium 

 with which it is united Ijy two short and comparatively wide ducts. Both speci- 

 mens were sexually mature males and considerable cjuantities of spermatozoa 

 occupied positions in the pericardial cavity, and at various points in the coelomo- 

 ducts. These last mentioned organs arise from the postero-lateral borders of the 

 pericardial chamber in the form of clearly defined tubes, whose cells are nearly 

 cubical in form and support an al)undance of large cilia. Bending sharjily inward 

 each becomes continuous with a canal, of much larger size and ditTerent structure, 

 that after extending forward for a short distance pursues an irregular coiu'se 

 opening symmetrically on each side of the rectum. The lai'ge non-ciliated por- 

 tion of the ducts is composed of rather low cells with well-defined, basally placed 

 nuclei, in which the chromatin exists in the form of a moderate number of sharply- 

 defined granules. In the more distal part of each cell is a sharply defined vacu- 

 ole, in which are one or two light greenish yellow bodies, having the appearance 

 of concrements such as occur in the kidneys of several molluscs. At various 

 places these are in the act of escaping through the ru])tLued or ilissolved end of 

 the cell or having become free are nndergoing a process of solution. Such an 

 appearance in the kidney of other molluscs would not in any way appear unusual. 



