ANATOMY OF THE OLAM. 227 
gills, where the embryos in some species partially develop. 
Impregnation probably takes place within the branchial 
: chamber, the spermatozoa being 
swept in with the respiratory 
current, and coming in contact 
with the eggs as they are dis- 
charged. 
An excellent general view of 
the relation of parts to the 
body-walls and shell may be 
seen by hardening a clam, or 
better a fresh-watcr mussel, 
Unio (see Fig. 155, D) in alco- 
el see rece Sy eet an aang a 
diameters, d, pedal ganglia , pedal Verse sections. A section can 
gl, Gi see teas comments ep OS floated off in water and ex- 
Seapets! wets tamiatony nents: amined with a lens, ‘The per- 
the pedal muscles.—Drawn by W. K. fect bilateral symmetry of parts 
will thus be seen. 
The above description will answer for the majority ot la-- 
Fig. 161.—Zime htans, flying through the water, its long numerous filaments ex- 
tended.—From Brehm’s “‘ Thierleben.” 
mellibranchiate mollusks ; in the oyster (Ostrea) or in Ano- 
