ENSIS DIRECTUS. 113 



1. Why does this animal not need a shell that is as heavy 

 and closes as tightly as that of Venus f Does it show the same 

 points regarding the valves (umbos, beaks, lines of growth, and 

 ligament)? Later, when the shell is removed, the large carti- 

 lage pit on the left valve will be seen. 



2. The ventral borders of the mantle lobes are united except 

 near the anterior end, where there is a space through which the 

 foot may be seen. 



3. The siphons are large and muscular and may be retracted, 

 as in the specimen that you are handling, or may be greatly 

 extended, as may sometimes be seen in aquarium specimens. 

 Why does Mya need larger siphons than Venus does? 



4. Pick up a specimen that has the siphons extended and 

 notice the powerful ejection of water. Is it ejected from one 

 or both openings? How is this accomplished? Of what service 

 can such jets be to the animal ? Why are powerful jets of this 

 nature of more service to Mya than to Venus ? 



Notice the cartilage in the cartilage pit on the left valve. 

 What function does it perform? Why is there no need for a 

 large and powerful foot ? It is much easier to trace the alimen- 

 tary canal and the ganglion connectives in this form than in Ve- 

 nus? 



Belding: The Mollusk Fisheries of Massachusetts. Mass. Fish and Game 



Com., 1909. 

 Kellogg: Life-History of the Common Clam, Mya arenaria. Bui. U. S. 



Fish Com., 1899. 

 Mead and Barnes: Observations on the Soft-shell Clam. Rhode Island 



Com. Inland Fish., 20 to 24, 1900 to 1904. 



ENSIS DIRECTUS. (Razor-hell Clam.) 



This species is another representative of the order Eulamel- 

 libranchia and is introduced because of its adaptation for a 

 burrowing habit, and because of the great ease with which its 

 nervous system can be studied. Individuals are not uncommon 

 on mud- or sand-flats from which the water flows at low tide. 

 They may sometimes be seen protruding above the surface of 

 the mud, but are hard to approach because of their great sen- 

 8 



