• REACTIONS OF BIVALVE MOLLUSKS 305 



A. Qualitative Tests with Different Species 



1. Classification Based on Sensitivity 



Nagel ('94) has called the sense by which animals distinguish 

 changes in light intensity "der photoskioptischen Sinn," or 

 light-shadow sense, and speaks of animals as being "photoptic" 

 or "skioptic," when they are sensitive respectively to increases 

 or to decreases in light intensity. He gives a list of over 20 

 species of bivalves that he has tested and divides them into 

 6 classes, grading from purely skioptic through photoskioptic 

 to purely photoptic. I find it more convenient to group the 

 species tested at Woods Hole into 3 classes, viz. : 



a. Sensitive both to in- b. Sensitive to decrease c. Sensitive neither to 



creases and decreases. only. increases nor decreases 



1 . Mya arenaria 1. Pecten gibbus (var. 1. Cumingia tellinoides 



2. Ensis directus borealis) 2. Yoldia limatula 



3. Venus mercenaria 2. Ostrea virginiana 3. Solenomya velum 



4. Petricola dactylus 3. Modiola demissus (var. 4. Tellina tenera 



5. Spisula solidissima plicatula) 5. Nucula proxima (?) 



6. Callocardia morrhuma 4. Modiola modiolus 



5. Mytilus edulis 



6. Liocardium mortoni 



7. Anomia simplex (?) 



A question mark is placed after Nucula because the animal is 

 so small and the siphons so short that slight movements may 

 have been overlooked. So far as could be observed, however, 

 there were no reactions. Since Nagel ('94) puts Tellina com- 

 planata and T. nitida rather high in the photoskioptic class, it 

 may be that my failure to get responses from T. tenera was due 

 to unfavorable physiological states of the animals experimented 

 with. There is also some question about the position of Anomia 

 simplex, for, although the animals tested reacted only to- de- 

 creases, it is probable that the change from a depth of 8 to 10 

 meters — the level at which they were collected — to the surface 

 caused a very great decrease 'n physiological tone. Such a 

 loss in physiological tone was especially noticeable in Pecten 

 and Area collected from similar depths. In fact, Area failed to 

 give any consistent responses. I failed to find any species which 

 reacted to increases only. Some of those in class a reacted more 

 strongly to increases than to decreases, e.g., Venus, but all except 

 those in class c reacted to decreases. 



