30 /. H. Poii'crs 



posed to the light, and with the exception of a single lazy lubber 

 (which grew much larger and more robust) they were in almost 

 ceaseless movement. Still, I could not account for the full differ- 

 ence in their forms and more especially in their heads. One of 

 the adults from these laboratory specimens is figured as figure i 

 on plate VI. It will be seen to be rather slender in outline and 

 especially narrow-headed. Yet the whole animal was very 

 rounded and plump, much more so than the pond adults with 

 heads of twice the bulk. The difference, too, seemed to be en- 

 tirely in the build of the animal, not in accessory structures like 

 the parotids. Finally, the most astounding difference proved to 

 be in the gape of the mouths. Those from the laboratory were 

 much smaller than I had ever seen in A. tigrimim before. When 

 chloretoned and completely relaxed, I could still scarcely open 

 the mouth wide enough to see the palatine teeth. The explanation 

 of this marked contrast was at first not evident, as all these ani- 

 mals had developed on the same food from the beginning. 

 Finally, it occurred to me that the constantly increasing differ- 

 ences might be correlated with differences in the manner of feed- 

 ing. The water of the pond was so muddy that the annuals were 

 in nearly total darkness, and their prey, though abundant, was not 

 massed. How did the animals under such circumstances manage 

 its capture ? 



I placed a number of the pond larvae in a large tank in a base- 

 ment without windows and supplied this tank moderately with 

 daphnids. By cautiously raising a dim light above the margin of 

 the tank I was able to catch the animals feeding in apparently" 

 their characteristic manner. The daphnids were evenly distrib- 

 uted throughout the water, and the larva, opening the mouth and 

 expanding the whole buccal cavity to the utmost, lunged vio- 

 lently forward, trusting evidently to the chance catch of as many 

 daphnids as the large mouthful of water might contain. The 

 straining of the whole buccal and branchial apparatus was very 

 noticeable, and the whole process seemed to be repeated as ryth- 

 mically as are the swimming movements of a medusa. 



Repairing now to my laboratory jars, I observed tlie feeding 

 of my larvae in the light upon their nuich more concentrated food 



226 



