ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 43 1 
surrounding it. The nucleus is seen in the basal end of the cell and is surrounded by 
dense protoplasm. Certain cells show a constriction below the vacuole, as if they were 
in process of being divided. Other cells show this process farther advanced, and appear 
as if they were drawn out by some force which was stretching them into two. The 
nuclei in these cells 
are to be seen in the 
basal half, and it ap- 
pears also that very 
little cytoplasm is 
thrown off with the 
vacuole. This 
method of excretion, 
although uncommon, 
is not especially 
wasteful, as would 
appear from the state- 
ments of investigators 
who have written 
upon this subject and 
maintained that the 
entire cells are ex- 
creted in the mollusks studied by them. Of course this may take place in some; 
but excretion in Atrina is not of that wasteful character. 
DIGESTIVE TRACT. 
The stomach is a large asymetrical pouch which adheres closely to the dorsal wall 
of the visceral mass. On the left side it is attached to the ventral wall by a strand of 
muscle tissue. The ducts of the liver 
open into it at two points, one on the 
right and one on the left. (Fig. 9, d.) 
A part of the epithelium lining the roof 
of the stomach is differentiated as a 
conspicuous gland which forms a prom- 
inent ridge within the cavity of the 
stomach. The anterior end of the crys- 
talline style adheres closely to this gland. 
The intestine originates at the pos- 
terior end of the stomach and passes to 
the extreme posterior end of the visceral 
mass, where it bends sharply to the right and passes anteriorly as far as the stomach. It 
then makes a large loop and again passes posteriorly, traverses the ventricle and mantle 
gland, to end behind the adductor muscle (Fig. 9.) 
The part of the intestine nearest the stomach possesses a feebly developed typhlosole, 
while the remainder has it very strongly developed. A large crystalline style lies in 
Fig. 7. — Bodies excreted from the kidney, formed by the pinching off of the vacuolated tips of 
the cells. Note the tuft of long cilia on each and the concretions of waste material within 
the vacuole. 
