126 • MOLLUSC A. 



Place specimens in dishes of sea-water, and when they have 

 opened their shells notice: 



3. The mantle. See if it is sensitive. How far can it be drawn 

 back into the shell? What muscles are used in withdrawing it? 

 Why is it necessary to withdraw it? What is peculiar about the 

 shape of the margin? What reason is there for this structure? 



4 The bright specks, the pallial eyes, along the margins of 

 the mantle. Are they placed in any order? 



6. The arrangement of the tentacles on the margins of the 

 mantle. Why should sense organs be placed in this position? 



6. 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 inter-filament ar junctions are near the inter-lamellar 

 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 so they can be drawn together when the 

 animal swims. 



Drawings to show the arrangement of the organs and the strvc- 

 ture 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 VIRGINIANA. (Orster.) 



This also belongs to the order Pseudo-lamellibranchia. It 

 forms a good example of adaptations 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. 



