GLOCHIDIA — FACTORS UNDERLYING ENCYSTMENT 483 



2. Axe-head glochidia. Proptera (Lampsilis) laevissima was 

 tested merely with KCl in order to compare its relative sensitivity 

 to this most active salt. There is little difference between the 

 reaction threshold of Proptera laevissima and Lampsilis luteola. 



KCl: 



N/100. All close promptly. 



N/250. Some contract at once, the rest progressively during an 

 interval of one to two minutes. 



3. Hooked glochidia. For comparison with the hookless and 

 axe-head types, the effect of KCl was tried on Symphynota 

 complanata, Arcidens confragosus, and Anodonta corpulenta. 



KCl: 



N/100. All three glochidia were thrown into snapping contractions, 

 some closing at once, others after several preliminary snaps. A certain 

 number closed progressively by a series of jerks. Anodonta displayed 

 by far the most vigorous snapping. 



N/250. Symphynota and Arcidens closed rather promptly. The 

 response in Anodonta was slower, perhaps due in part to retarded pene- 

 tration through a certain amount of undissolved slime which encloses 

 the larvae and is more resistant than that of the other forms. 



N/500, Symphynota closed in about two minutes, Anodonta more 

 tardily. 



N/1000. Symphynota: After rapid pulsations all closed within two 

 minutes; it is plainly more sensitive than the others. Arcidens: Some 

 become shut in one to three minutes and nearly all within five minutes. 

 Anodonta: Contractions are rapid and prolonged. At the end of five 

 minutes many are closed, whereas others continue snapping. 



N/2000. Symphynota: Some respond quite promptly; most of the 

 remainder are shut in five minutes. [Distilled water, by itself, is with- 

 out effect.] 



It follows from these observations that the threshold of 

 stimulation to KCl in hooked glochidia is far lower than in either 

 the hookless or axe-head types. Doubtless diminished responses 

 would have continued to considerably greater dilutions had 

 trials been made. Yet N/2000 KCl is physiologically very 

 dilute; it represents a 0.0037 per cent solution. The minimal 

 concentration of KCl effective in the sensory activation of certain 

 (marine) animals are (Arey and Crozier, '19): Ascidia, N/4; 

 Chromodoris, N/10; Synaptula, N/40; Chiton, N/160; Bal- 



