VENUS MERCENARIA. 91 



Order 1. Decapoda. 



Ten arms, two of which are elongated, suckers 



on stalks. (Loligo, Sepia, Spirula.) 

 Order 2. Octopoda. 



Eight arms, suckers sessile. (Octopus, Argo- 



nauta.) 

 Subclass 2. Tetrabranchiata. 



Tentacles numerous. External chambered shell. 



Funnel open along one side. Only one living 



genus. (Nautilus.) 



LAMELLIBRANCHIATA, 



VENUS MERCENARIA. (Quohog.i) 



Animals of this species wander around over muddy bottoms 

 in rather shallow water, keeping the siphon end, at least, above 

 the surface of the mud. If possible, you should find specimens 

 in their native places and watch their movements. Specimens 

 placed in water and left undisturbed for some hours are likely 

 to protrude the siphons, and the foot may be protruded in some 

 cases. ^ Allow powdered carmine to slowly settle past the open- 

 ings of the siphons and determine the direction of the current 

 of water for each. Touch portions of the animal and find what 

 parts are most sensitive. 



Shell. — Note its general shape, and that it is composed of 

 two symmetrical parts, the valves. For each valve notice : 



1. The outline. 



2. A swelling, the umbo, ending in a point, the beak, from 

 which growth has proceeded. 



3. The lines of growth. Were the valve cut off along one of 

 these lines, the shape would not be changed. Why are the lines 

 arranged in this manner? How were they formed? 



The two valves are joined by the ligament. The margin 



^ Points in which the fresh-water mussel differ have been noted, so the 

 directions may be used for that form. 



^ Other species of lamelhbranchs are more satisfactory than Venus for 

 studying movements, as they expand quickly after being disturbed. Among 

 the common ones that may be mentioned are Ensis, Cumingia, Yoldia, 

 and Mytilus. 



