Microns 



100 



Figure 88. — Transverse section of the periostracal groove. The cells on the left side are distended with secretion; those 

 at the very bottom are short, almost cuboid. The black mass at the bottom of the groove is conchiolin. Bouin, 

 hematoxylin-eosin. 



Owing to the presence of the conchioHn-secreting 

 cells, the entu-e outer surface of the mantle is 

 sticky and adheres closely to tlie inner surface of 

 the sliell. List (1902) advanced a theory, not 

 well supported by observation, that in the 

 Mytilidae the mantle adheres to the shell by 

 means of fibrillae which originate in the myoepi- 

 thelial cells and pass tlirough the epithelium. Such 

 an arrangement is not found in ('. virginica or in 

 C angulata, and according to Leenhardt (192()) 

 does not exist in Mytilun. 



The epithelial layer along both surfaces of tlie 

 mantle including its free edge contains alkaline 

 phosphatase, an enzyme involved in the calcifi- 

 cation of the shell. The presence of the enzyme 

 can be demonstrated by the Gomori method 



(Gomori, 1939, 1943) based on the formation of 

 insoluble calcium phosphate as a result of phos- 

 phatase action on sodium glycero-phosphate and 

 calcium ions. Furtlier treatment with 5 percent 

 silver nitrate (or with cobalt nitrate) converts 

 tlie calcium phosphate to silver (or cobalt) phos- 

 phate which turns black after exposure to light. 

 Both reagents gave satisfactory results in demon- 

 strating the localizatiiin of the enzyme in the 

 epithelium of the mantle. Tlie strongest reaction, 

 judgetl by the opacity and width of the black 

 layer, was ftnmd to occur along the edges of the 

 mantle and in the area of the periostracal groove. 

 Even the tips of the tentacles contained noticeable 

 amounts of the enzyme (fig. 89). These obser- 



88 



FISH AXD WILDLIFE SERVICE 



