SEA MUSSEL MYTILUS EDULIS. 1 69 



HISTOLOGY. 



The cells which make up the kidney tissue are of two types, as described by Sabatier 

 (1874). One class is found in what he termed the free or independent part, as is best 

 seen in the pillars of the kidney and the membrane which separates the kidney canal 

 from the pericardial cavity. These form a pavement epithelium of nearly cubical cells, 

 with the free edge presenting a convex surface. A large, round nucleus lies in the center 

 or near the base of the cell. The cytoplasm is clear and transparent and often contains 

 either few or numerous small greenish granules. In size these cells vary from 8 to 

 10 microns in diameter. 



The second type of cell is found in that portion of the kidney which lines the wall 

 of the veins. They form a columnar epithelium in which the cells may vary from 5 to 

 24 microns in height with a diameter of from 4 to 6 microns. The nucleus lies near the 

 base of the cell. Between the nucleus and the distal end of the cells the transparent 

 protoplasm is filled with numerous fine granules in which are often seen one or two 

 large roundish bodies which are apparently nuclear concretions of a crystalline nature. 

 The free edges of the cells present a decided convex surface. 



The structure of the pericardial glands has been most thoroughly worked out by 

 Grobben (1888). He pointed out that the glands consist of numerous small flaps or 

 folds of different sizes which are dark brown in color and that they completely surround 

 the auricles. The cells are of different shapes and sizes, some being low and broad, 

 while others are tall, narrow, and cylindrical in form (fig. 155, opp. p. 169). The cells in 

 most cases are more or less separated from each other, and where connection does exist 

 between them it is in the basal region only. Exception to this rule occurs in the case 

 of the very tall cylindrical cells, which are often so thickly crowded together that their 

 entire side walls are in direct contact with those of the neighboring cells. Spaces, how- 

 ever, are often observed between them. The outer ends of the cells are convex and 

 bear on the apex a single vibrating flagellum (fig. 156, opp. p. 169). The flagella are 

 visible on the living cells, but when prepared histologically they disappear. 



The cells contain nuclear concretions of various sizes and forms near their periph- 

 eral ends. In color these particles are brownish-green to black and highly refractive. 

 It is to their presence that the dark shade of the gland is due. Sections through the 

 glands show that the concretions are larger and more abundant in the cells lying in the 

 deeper invaginations. 



Grobben also occasionally observed pale, spherical bodies lying close to the refrac- 

 tive concretions and, in the deeper invaginated parts, vacuoles were found present in 

 the cells (fig. 155). Whether or not the vacuoles exist in living cells the author was 

 unable to determine. 



In the lumen of the invaginated folds epithelial cells richly laden with concretions 

 are often seen cut off from the epithelium (fig. 155). They contain their concretions 

 molded either into a single large sphere almost the size of the cell body or as several 

 large concretions which make up the greater part of the cell contents. These leave the 

 pericardial cavity whole or in a disintegrated form by way of the opening to the kidney. 



PHYSIOLOGY. 



The function of the lamellibranchiate kidney and of the pericardial glands has been 

 studied by Kowalevsky (1889), Letellier (1891), and Cuenot (1899). Their method in 



