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 Serobicttlaria, 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 

 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- 

 cally it is the survival of the posterior adductor. 



THE BRANCHLE OR GILLS 



A : 1, incurrent siphon ; 2, excnrrent siphon ; 

 3, foot. B: 1, siphons ; 2, pedal opening. 



Removing carefully the thin, fleshy mantle, trie gills are ex- 

 posed to view that is, of course, the gills of that side of the 



