104 MOLLUSCA. 



MY A ARENARIA. (Long Clam.) 



This animal belongs to the order Eulamellibranchia, as does 

 Venus, and is introduced because of adaptations for its manner 

 of living. It lives buried in the mud, in which as an adult it 

 remains stationary. You should find a "clam bed" along the 

 shore, and after noticing the pits in the surface of the mud, and 

 the jets of water that are sometimes thrown from the pits, dig 

 down and see how the animals are placed. If the water is calm, 

 see if you cannot find the openings of the siphons at the surface 

 of the mud, of specimens that are still covered by water. You 

 will need to walk very carefully so as to disturb mud and water 

 as little as possible, as the siphons are otherwise closed and 

 withdrawn. 



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

 and closes as tightly as that of Venus? 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? 



