PELECYPODS 413 
The figures on pages 411, 412, and 413 show the degrees of man- 
tle fusion in various groups from the open to the almost wholly 
closed types. One (A, page 413) gives an example of very large 
siphons united together and protected by the tough, leathery in- 
tegument. To the right of this cut may be seen the foot project- 
ing from the pedal opening in the mantle. Another gives the 
two siphons of Cardium, the lower or branchial one being the 
longer. The mantle-edges below are fluted, and a large foot pro- 
jects forward. Another shows two siphons of Scrobicularia, of the 
free and greatly extended type. Fig. B (page 413) illustrates the 
ventral portion of a pelecypod, which shows very well the most 
advanced type of mantle fusion. 
Besides the siphonal openings 
at the end of the long projec- 
tion to the left, the only other 
opening is the very small orifice 
marked “2,” for the accommo- 
dation of a small, insignificant 
foot. In the other figures dif- 
ferent degrees of mantle fusion | 
are indicated. 
Before removing the mantle — 
from one side of the specimen , #),> BT uphowe; fpelsl opting n 
to be examined, note the two 
strong muscles which are at either end of the body. These are 
the anterior and posterior adductor muscles, their office being to 
hold the valves tightly shut. They are of great strength, as any 
one may prove to his satisfaction by attempting to open a clam. 
Through a long series of forms these two muscles approach each 
other in position, and in the scallops and the oysters there is but 
one adductor muscle, occupying a central position. Morphologi- 
eally it is the survival of the posterior adductor. 
THE BRANCHIZ OR GILLS 
Removing carefully the thin, fleshy mantle, the gills are ex- 
posed to view—that is, of course, the gills of that side of the 
