MUSSELS AND SNAILS. 
157 
of the gills. How many do you find ? The mouth is 
between the palps. 
Examine the ventral and dorsal passages of the 
siphon. Are they connected ? Into what cavity does 
each lead ? 
See if you can find the heart near the dorsal margin. 
Have you found that the mussel has a head ? What 
organs of sense have you found ? 
Examine mussel-shells which have been burned for 
some hours in a hot fire. Compare with them some 
Fig. 123.—A Fresh-water Mussel ( Anodon ) Showing Position of Foot 
and Siphons. 
shells which have been immersed in weak acid for 
several days. Examine, also, oyster-shells which have 
been prepared in the same way. Describe the structure 
of a shell. 
On the dorsal margins of the valves of a shell find 
