OSTREA VIRGINIANA. SOLENOMYA. 103 



OSTREA VIRGINIANA. (Oyster.) 



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. 



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 completeness 

 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? 



3. Open the shell by breaking the edge, inserting a knife- 

 blade through the opening, and cutting the adductor muscle 

 away from the flattened left valve of the shell and notice the 

 single adductor, extensive gills, and the absence of a foot. 



SOLENOMYA. 



This form, a member of the order Protobranchia, with much 

 the same structure as Yoldia, shows an interesting method of 

 swimming that should be compared with Pecten, and with the 

 jets of water formed by Mya. Specimens may be dug at low 

 tide from mud or sandy mud, placed in a dish of sea-water, 

 and observed. Does the posterior opening in the mantle cham- 

 ber correspond to typical siphons? See if you can find how the 

 animal swims. Is the movement continuous or jerky? Does the 

 animal move forward or backward? Is the foot active? Are 

 jets of water thrown from the shell? Is the animal adapted to 

 forming jets of water? 



Examine a specimen that has the valves closely drawn to- 

 gether and see how rounded the margins appear. Examine a 

 shell from which the animal has been removed by maceration and 

 see the relation of the shell cuticle and the calcareous portion of 

 the shell. What becomes of the marginal cuticle when the shell 

 is closed? Can this have anything to do with throwing jets of 

 water from the shell ? 



