1908.] ASPIDOBRAXCH GASTROPOD MOLLUSCS. 867 



shown in fig. 54. It is interesting as exhibiting in its simplest 

 form a mixed epithelium consisting of ciliated and glandular 

 cells. The latter are very large with basal nuclei surrounded by 

 dense granular protoplasm, the outer ends of the cells containing- 

 eosinophilous granules in a protoplasmic reticulum. The ciliated 

 cells are very small and wedged in between the glandular cells in 

 a very regular manner. The pi-ostate cori-esponds to what 

 Lenssen calls the " giande annexe " in jS^. Jiuviatilis, in Avhich 

 species it is i-elatively of enormous size. Lenssen did not observe 

 that the epithelium is mixed, as just described, but though it is. 

 not so easy to distinguish its characters in the former, there is no 

 difference between N. Jiuviatilis -awA Parmierita in this respect. 



The thalamus, then, is a split-like cavity extending some- 

 distance in front of and behind the entrance of the sperm-duct. 

 Below and at the sides it communicates widely with the vast 

 cavity which I shall call the teiminal chamber. Lenssen has. 

 called it the "poche semilunaire " from its appearance in section. 

 This is in reality a large pyrifoi-m sac, of which the outer 

 wall remains thin and but slightly glandular, while the inner 

 wall is greatly thickened by glandular differentiation of its- 

 epithelium and projects like a demi-column into the cavity of the 

 •sac, reducing its lumen to a crescentic slit (figs. 44, 48, & 50). 

 This projecting glandular column is attached along a slightly 

 spiral line, is much thicker posteriorly than anteriorly, and its 

 hinder end loses its attachment to the inner wall and projects- 

 backwards into the cavity of the .sac. Hence sections through 

 this end show not a semi-lunar but a circular cavity surroi.inding 

 a central pillar (fig. 49). The thalamus opens into the recess- 

 where the column becomes free from the inner wall. Anteriorly 

 the terminal chamber diminishes in diameter ; its walls become 

 less glandular and more muscular, but the glandular thickening 

 on the inner side is continued for some distance forward and 

 eventually ends in a free projecting process, not far from the 

 external aperture. 



Posteriorly a gland of some size (figs. 48 &, 50, h.gl.) opens into 

 the terminal chamber. This like the prostate has a distinct 

 lumen, and is a saccular outgrowth of the hinder part of the 

 terminal chamber with much -folded walls. Its epithelium is- 

 wholly glandular, without any admixture of ciliated cells, and the 

 elements composing it are loaded with globules which stain 

 deeply in hfematoxylin and are therefore probably mucinogenous^ 

 Each globule contains a minute spot, .staining brightl}^ in carmine,, 

 and the Avhole epithelium has a very characteristic appearance., 

 which I have represented in fig. 56. 



That the terminal chamlDer corresponds to the ootype of the 

 female there can be no doubt. The structure of the glandular 

 walls is identical. The outer wall is only feebly glandular compared 

 with the inner, but its structure, shown in fig. 52, is interesting as 

 illustrating the steps by which a mixed ciliated and glandular 

 epithelium, such as that of fig. 54, has become modified into the 



