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 ina hot fire. Compare with them some 
Fic. 123.—A Fresh-water Mussel (47odonx) 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 
