GLOCHIDIA — FACTORS UNDERLYING ENCYSTMENT 477 



B. Photic excitation 



To determine whether conditions of illumination influence 

 glochidial activity, ripe individuals of Lampsilis luteola were 

 divided into two lots, one of which was subjected to darkness for 

 seven hours, the other to bright, diffuse daylight for a similar 

 period. Tactile tests revealed no difference in sensitivity be- 

 tween the two. Similar negative results have followed trials 

 after illumination with strong and with weak light. Glochidia 

 of several Lampsilis species were found not responsive to dif- 

 ferences in light intensity through shading; this applies both to 

 quiescent larvae and those in an irritable, 'snapping' condition. 



There is a feeling among some experienced in mussel propaga- 

 tion that bright sunshine favors infections. Thus Howard 

 ('14, p. 21), referring to the difficulties of artificial infection with 

 Quadrula ebenus, writes: ''The results were more favorable when 

 the sun was out than during cool, cloudy weather and apparently 

 better in sunhght than in shade." So far as they go, the ex- 

 periments which I performed with the Lampsilis group do not 

 support this view; yet the suspicion arises that if there really are 

 better results with illumination, as described, they are referable 

 to temperature rather than light. Incidentally, the conditions 

 of infection in nature are not such that a correlative sensitivity 

 to quantitative light differences would be generally useful. Dr. 

 F. H. Reuling states that in his experience with the summer 

 propagation of various lampsilids he has never been able to 

 establish any correlation between ordinary diurnal variations 

 of light and temperature, on the one hand, and the ease of ob- 

 taining infections, on the other. 



C. Thermal excitation 



Glochidia of Lampsilia luteola, cooled to 10° to 15°C,, showed 

 no perceptible decrease in tactile response. When the temper- 

 ature was reduced to 3° to 5°C., the reactions were apparently 

 duller and less vigorous. It is entirely probable that a time 

 factor operates here; there was some evidence obtained to sub- 



