158 ANIMAL ACTIVITIES. 
the hinge-teeth if there are any. How many teeth do 
you find on each shell ? 
Compare the shell of the common salt-water clam 
(Mya Arenaria) with that of the mussel (Cxz0). How 
do the teeth and hinge-ligament differ ? 
How do the markings on the inside of the shell differ ? 
What differences in the structure of the animals do 
these markings indicate ? 
In the same manner compare the shell of oysters, 
pectens, and other bivalves. 
Write a description of a shell you have not previously 
seen and compare your description with that given in 
Woodward’s Mollusca or a similar book. 
Summary of Drawings. (a) Sketch of a living 
mussel showing foot and siphons. 
(4) Cross-section of valves showing action of hinge. 
(c) Inside of shell showing all muscular impressions 
and the pallial line. 
(2) Sketch of mussel in its shell. 
(e) Cross-section of the body of the mussel near the 
umbo. 
(/) Shells of several bivalves. 
Activities of the Clam or Mussel. Feeding. We 
have already noticed currents of water flowing in and out 
of the siphon at the posterior end of the body. The 
water enters by the ventral opening and goes out by 
the dorsal opening. The dorsal and ventral passages 
are separated, and this separation continues through 
the body of the clam, dividing it into two chambers, 
the dorsal or cloacal chamber and the branchial or gill- 
chamber. In many marine clams the mantle-edges 
unite again below the branchial siphon, but in the 
fresh-water clam they are free along the ventral border. 
As water enters the lower siphon it brings with it food 
and oxygen into the branchial chamber. In_ this 
chamber hang the two pairs of ridged leaf-like gills 
bearing cilia which by their movements keep the cur- 
rents of water in motion. These cilia also select from 
