ON THE STRUCTURE OF ENIGMA .ENIGMATICA. 285 



of what would be the case if they were " little rods of 

 secreted material." 



Between the epithelial walls of the lamina is a plexus of 

 connective-tissue cells, among* which there are elongate pyri- 

 form or spindle-shaped masses of granules in which no nuclei 

 can be distinguished. Deeper down in the laminae a few 

 glandular cells loaded with granules are scattei'ed through 

 the connective-tissue core, but there is no compact mass of 

 byssogenous cells, such as is usually to be found in other 

 Lamellibranchia. The elongated strings and globules of 

 granules must be identified with the streams of granules 

 which I hiive described in Jousse aumia (3) as travelling by 

 intercellular paths from the byssus gland to the byssus cavity. 

 I am of the opinion that the byssogenous cells break up, and 

 that the secretion travels between the irregular spaces of the 

 connective tissue, and that there are not definite ducts as 

 described by Horst (7) in Dreissensia. Boutan figures 

 irregular branching ducts in Area tetragona (4, pi. 13, 

 fig. 12), but he does not enter into histological details, and 

 bis figure might equally Avell be interpreted according to 

 Horst's views or ray own. Perhaps the most remarkable 

 feature in y^nigma is the cap of granular columnar cells on 

 the edge of each byssogenous lamina. These cells are clearly 

 continuous Avith the ciliated epithelium of the sides, but they 

 are not ciliated, and are filled with byssogen granules. It 

 may be inferred that they have taken up these granules from 

 the intercellular channels of the connective tissue, and that 

 they secrete them again at their free surfaces, thus adding to 

 the thickness of the byssus plate. The concentric lines in 

 the latter (fig. 18) clearly indicate that there has been a con- 

 tinuous addition of fresh matter from the large granular cells 

 capping the edge of the lamina. There is no possibility of 

 confusingthebyssogenous with the mucous cells in Enigma ; 

 the latter are, indeed, numerous in the foot and in the lips of 

 the byssus cavity, but they never penetrate into the laminae, 

 and are easily distinguished by their oval or polygonal shape 

 small nuclei and clear contents. 



