l^HE LIFE-HtS'J'ORY OJ^' NUOULA bELPHJNODONTA. 347 



acquires its fifth pair of plates (tig. 45), tlie byssal gland 

 extends through a considerable portion of the foot, and in 

 the posterior side of the foot it may extend to a position 

 somewhat dorsal to the pedal ganglia. The cells of the gland 

 during this stage are greatly swollen and vacuolated, and 

 have thin, almost indistiuguishable walls. They are crowded 

 together so as to almost obliterate the lumen of the gland. 

 The result is that stained sections of the gland have the 

 appearance of a fibrous or reticular mass that is so mixed 

 up as to be hardly intelligible. 



As the embryo gill begins to acquire its fifth pair of plates 

 the byssal gland generally becomes less extensive. In the 

 adult it is reduced to a small pouch (fig. 48, hg.) that opens 

 in the median groove of the foot, just anterior to the heel- 

 like projection. The dorsal, blind end of the pouch consists 

 of comparatively large cells with small nuclei, and seems to 

 contain some secretions. They are not generally distended 

 with secretion, and the duct is generally quite empty. Nothing 

 comparable to byssal threads have been observed. Towards 

 the opening of the gland the cells become smaller and bear 

 cilia. 



I have described the adult condition that seems most fre- 

 quently to prevail. In a few specimens the gland cells are 

 much shrunken, and seem to contain little or no secretion. 

 In some specimens of Nucula proxima the gland is more 

 extensive and the cells are greatly distended. This would 

 seem to indicate that the gland is functional, but not as an 

 organ for the formation of threads. The present use of such 

 a secretion is problematical. 



It is very natural to compare this gland to the mucus- 

 secreting glands of Gastropoda, but there seems to be little 

 direct evidence that they are homologous. 



Alimentary Canal. 



There is a stage when the embryo resembles an epibolic 

 gastrula (fig. 4) . A pouch appears between the large cells, at 

 a point corresponding to the asterisk, that seems to be formed 



