1908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 851 



modified epithelium forming the bulk of the gland. The duct is 

 short and wide, and as it enters the ha^moccelo is accompanied by 

 a stout strand of muscle, from which flat musculai' partitions 

 radiate in all directions. The lumen of the duct enlarges greatly,, 

 and forms a sort of sac the cavity of which is broken up into a laby 

 rinth, formed by the ingrowth of the muscular partitions carrying 

 before them the highly modified glandular epithelium of the sac. 

 The whole forms a large and compact organ lying below the 

 viscera, and very conspicuous when the animal is disseHed. The 

 structure of the epithelium is peculiar (fig. 34) and sections of 

 it are not easy to interpret. Judging from the transitional 

 epithelium lining the duct, it appears that the ordinary cylindrical 

 epithelial cells become very slender and are compressed by the 

 excessive development of the gland-cells lying between them : 

 they lose their cilia ; their nuclei, reduced to a small size, stain 

 very deeply and ai^e shifted to the external ends of the cells, the 

 cytoplasm radiating outwards from the nuclei to form a cover 

 over the distal ends of the adjacent gland-cells. They have in 

 fact undei-gone a modification similar to that of the cover-cells of 

 the testis of Hydra. The gland-cells appear to be of two kinds : 

 clear cylindrical cells with finely granular contents staining 

 faintly in hfematoxylin, and cells of similar shape but filled with 

 irregular and highly eosinophilous globules. I have satisfied 

 myself, however, that these are only different phases in the 

 secietory activity of the same kind of cell. The pale, finely 

 granular cells represent the resting condition. Their nuclei are 

 very small and deeply staining and are either close to the base 

 and on one side of the cell or somewhat higher up and close to 

 one side. Some of these cells are vacuolated, their contents stain 

 lightly in eosin, and their granular contents begin to aggregate 

 into globules, this process generally commencing at the basal end 

 of the cell. In later stages the whole cell-body is filled with 

 eosinophilous globules of irregular size, and these are finally 

 discharged, by rupture of the cover-cells, into the lumen of the 

 gland. The space between the hinder flap of the mantle and the 

 opercular lobe is commonly full of the sticky coagulated secretion 

 of the gland, but what its function is I am unable to guess. 

 From its large size, the opercular gland must be of considerable 

 physiological importance, but it is remarkable that it only occurs 

 as a highly developed and differentiated structure in marine 

 foimis. Possibly its secretion is useful in assisting the animals to 

 retain their hold on rocks washed by the waves. 



The Alimentary Tract. 



This may be treated very briefly, as the structure of the buccal 

 bulb, oesophagus, stomach, and intestine is very constant through- 

 out the family, and the details in which the various species differ 

 from one another are so small and unimportant that they may be 

 passed over. A sketch of the alimentary canal of Faranerita 



