ON THE SEXUAL PHASE IN CERTAIN OF THE NAIDID^I. 791 



where it joins the ampulla ; it is not as long as the ampulla, and opens on the surface 

 in front of the ventral setae of segment v, about the position of septum 4/5. 



The alimentary canal in these specimens showed the following very curious 

 condition (fig. 3). 



There was no mouth opening ; the epithelium clothing the anterior lip turns in 

 at the site of the mouth and can be followed for a short distance as a definite layer 

 of cells (the former anterior or dorsal wall of the buccal cavity) ; the epithelium of 

 the posterior lip, however, fuses with this layer without itself turning inwards. 

 The buccal cavity is no longer a cavity, but a sheet of cells for the most part 

 disintegrating. 



The pharynx is represented by a mass of cells without epithelial arrangement, 

 with spaces between them, but no continuous lumen ; many of the cells are disinte- 

 grating ; the mass is enclosed within a basement membrane, outside which are a few 

 peritoneal cells. Posteriorly and dorsal to the remains of the pharynx is a mass of 

 large round cells, a large part of each being constituted by the nucleus. 



The gut is continued back from the remains of the pharynx as a narrow fibrous 

 band, dorsal to the nerve-cord and ventral vessel, without lumen, and in the genital 

 segments invested by a quantity of granular matter. 



Behind the ovisac, in segment xii, a lumen first makes its appearance. This 

 consists at first of a series of irregular isolated cavities, cut off from each other by 

 degenerating masses of cells ; the whole width of the intestine is much less than 

 under usual circumstances. Further back, in the middle of the body, the gut 

 possesses a continuous lumen ; the lining epithelium is much altered and highly 

 degenerate ; cell outlines are not to be made out, and the epithelial layer consists of 

 a thin layer — much less in height than the normal columnar cells — of cytoplasmic 

 material with occasional nuclei. The cavity is filled with a semi-transparent, colour- 

 less, finely granular mass. 



This condition is continued to the posterior end of the body, where the lumen 

 may again be absent. The gills are small, and appear much shrunken ; in sections 

 they have lost the definite and typical structure which characterises them in the non- 

 sexual individual. 



The cerebral ganglion and ventral nerve-cord are maintained in apparently their 

 normal condition ; and so also the nephridia and blood-vessels. That the animals 

 were normal and not pathological was shown by their powers of active locomotion ; 

 on being disturbed in the slightest they executed the most violent wriggling move- 

 ments, rather like those of some insect larvae. 



In looking through my collection of slides to see if I might confirm this curious 

 atrophy of the alimentary canal, I was fortunate enough to come on a series of 

 longitudinal sections of a sexual Dero, made a few years ago. The species was not 

 identified at the time, but it appears from my notes on it, and from the specimen 

 itself, not to be D. limosa. 



