26 



Another peculiar histological structure may be 

 referred to here. The epithelium of the lamellae is 

 simply the folded epithelium of the inner surface of the 

 mantle. The outer epithelium hounding the mantle exter- 

 nally is, of course, not folded here or elsewhere (fig. 31, 

 Pall. ep.). Between these two layers there is the very 

 characteristic median stratum of connective tissue made 

 up of extremely large cells with delicate walls, feebly 

 staining nuclei and practically no contents (fig. 31, 

 X. con.). What function this layer may play has so far 

 not suggested itself. 



MUCOUS GLAND. 



The mucous gland is the most conspicuous organ in 

 the pallial cavity, both by reason of its secretion as well 

 as by its structure. It is a modified region of the mantle 

 between the ctenidium and the rectum where gland cells 

 predominate, and the inner wall of the mantle is thrown 

 into deep folds which run transversely, overlapping 

 slightly and hanging down into the pallial cavity. 



The mucous gland is really the inner wall of the 

 mantle whose cells are elongated and specialised as gland 

 cells. The anterior folds of the gland are directed 

 backwards, the most posterior ones forward (fig. 8, 

 Mu. gl.). They are much thicker than the ctenidial 

 leaflets and are separated by much greater spaces. The 

 number varies, 20-25 being about an average for a full- 

 sized whelk. 



This region of the mantle is extremely well 

 supplied with blood, as a glance at fig. 35 will 

 show, and numerous vessels run down parallel to 

 the folds from the reno-mucous vessel to the afferent 

 branchial vessel. The secretion of the gland is either 

 perfectly hyaline or yellow-white in colour. It is 

 perfectly abominable to handle, and after months in 



