392 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
kind seems to be gradually transformed into another, and 
probably there is only one type of secreting cell present. 
The ducts (fig. 50) conveying the secretion to the 
stomach divide up in the gland into several branches lined 
with ciliated epithelium. The cells are columnar and 
granular in appearance, with a prominent nucleus near 
the base. They are supported by a layer of connective 
tissue—the tuneea propria (fig. 50, Tu. p.). Outside 
this, again, is a layer of circular muscle fibres, which pass 
round the duct forming the tunzca muscularis (fig. 50, 
Lu.m.) “* Macroblasts,”’ or eosinophilous cells, as seen in 
the oyster, are not present in Pecten, though under the 
action of the same fixatives and stains they show up 
strongly in the mantle. 
The only type of cell present appears to be the 
granular cell lining the alveoli (fig. 50, Ad.), but smaller 
ceils can be observed between some of these, which stain 
much more intensely than the others. ‘These are probably 
similar to the cells described by MacMunn (21) in the 
gastric gland of Patella, and are young cells which will 
replace the others, for, in sections through the alveoli, the 
granular cells can be seen in process of being shed into 
the lumen, and there are bodies in the course of the ducts, 
of common occurrence, that are undoubtedly these shed 
cells on their way to the intestine. In this way the 
digestive gland appears to have an excretory function 
in addition to its storing and pancreatic functions. 
Pigment concretions do not appear in the lumen of any 
of the alveoli. Cilia are confined to the ducts, and as an 
alveolus is followed towards its caecal end the cells become 
fewer and more swollen (fig. 5, Ad.') and intensely 
vacuolated, the nuclei lying against the connective tissue 
membrane supporting the cells. Mventually the lumen of 
the alveolus disappears altogether, and the cells meet in 
