864 PROF. G. C. BOURNE ON THE [jSTov. 17^ 



mantle and therefore lined by an ectodermic epithelium. I 

 iDelieve that the homologue of such an ectodermal ureter is to be 

 found in the structure which I have referred to as the uropore- 

 sac. As shown in fig. 45, the mantle-epithelium smu'ounding the- 

 uropore is modified, consisting of high columnar ciliated cells. 

 This epithelium passes round the lips of the uropore, and appears 

 to line the uropore-sac, which, however, should be described as a 

 recess rather than a sac, for it is widely open below into the non- 

 glandular part of the kidney, and only has the appearance shown 

 in fig. 47 in sections passing through its upper part. For some 

 reason the epithelium lining this recess is very much macerated 

 in all my specimens and I cannot give a clear account of it ; but 

 the thick wall appears to be made up of a number of glandular 

 crypts, or, rather, of bunches ,of gland-cells, which have passed 

 through the basement-membi-ane into the subjacent connective 

 tissue, as is the case with the various accessory genital glands. 

 The ciliated epithelial cells retain their position on the outside of 

 the basement-membrane. Lower down, in the vicinity of the 

 uropore, the glandular structure gradually disappears, but the 

 columnar ciliated cells are continued as a broad band running 

 back for some distance along the anterior and inner wall of the 

 bladder (fig. 46, cl.ep.). The histological characters of this 

 ciliated band leave little doubt that it is an ingrowth of the 

 ectoderm, and its function is obviously to create a powerful 

 ciliary current in the direction of the uropore. 



The position of the little folded organ which lias been referred 

 to as possibly representing the right ctenidium is indicated in 

 fig. 43, E.ct. In some specimens it is only represented by a 

 thickening and modification of the mantle-epithelium exactly like 

 that round the itropore. In other specimens the epithelium is^ 

 thrown into folds, but this may be due to contraction in spirit. 

 The mantle- wall is thickened by a special development of muscular 

 strands below the patch of modified epithelium, and in connection 

 with these is a blood-space, which is nothing more than a diverti- 

 culum of the large transverse blood-sinus enclosing the visceral 

 ganglion. The organ in question has, therefore, the same relations 

 on the right that the functional ctenidium has on the left to the 

 transverse blood-sinus. Furthermore, its position with regard to 

 the hypobranchial mucous gland is precisely that of the right 

 ctenidium to the right hypobranchial gland in dibranchiate 

 Rhipidoglossa. Hence, in spite of its distance from the rudi- 

 mentary right aui-icle and the fact that, so far as I am able to 

 determine, it receives no sjjecial nerve-supply, I regard it as 

 a vestige of the right ctenidium. The hypobranchial gland 

 varies considerably in size in the different species of Nerita and 

 Paranerita^ but its position is always the same. The mass of 

 secreting tvibules forming the body of the gland lie in the roof 

 of the right-hand side of the mantle-cavity, and chiefly in the 

 posterior part of the swelling formed by the genital comj^lex. 

 Their position in the male of Paranerita gagates is shown in 



