PECTEN, OSTREA 137 



the mantle. Why should sense organs be placed in this posi- 

 tion? 



4. Specimens in aquaria will often swim. If possible, 



notice how this is accomplished. 



Wedge the valves of a specimen apart and notice the 

 single large adductor muscle. What need has Pecten for 

 such a large adductor? Notice the foot and compare it with 



the foot of Venus. 



How are the gills attached to the body? What would be 

 the effect on the gills if they were attached to the mantle and 

 to each other, as in most forms, when water is ejected in 



swimming? 



Examine the structure of the gill and notice how much 

 larger the interfllamentar junctions are near the interlamel- 

 lar junctions than elsewhere. Near the margins of the gills 

 the junctions are frequently simple bunches of cilia, as in 

 Mytilus. Observe the muscular movements of the gills. The 

 gills of this form are muscular and can be drawn together 

 when the animal swims. 



Drawings to show the arrangement of the organs and the 

 structure of the gill are desirable. 



Belding: The Scallop Fishery of Massachusetts. Mass. Fish and Game 



Com., 1910. 

 Drew: The Habits, Anatomy, and Embryology of the Giant Scallop, 



Pecten tenuicostatus. Univ. of Maine Stud., No. 6, 1906. 



OSTREA VIRGINICA (Oyster) 



This also belongs to the order Pseudolamellibranchia. It 

 forms a good example of adaptation for a sedentary life. It 

 occurs, fastened to rocks and other shells, in positions where 

 it is much exposed to attacks of the enemies of lamellibranchs. 



1. Notice the difference in the size and shape of the 

 valves. Why is this desirable? 



2. Notice the thickness of the valves and the complete- 

 ness with which they come in contact when the shell is closed. 

 Would such a heavy or tight-closing shell be satisfactory for 

 the scallop or the razor-shell clam? 



