Zool.— Vol. I.] HITTER- D I EMYCTYLUS TOROSUS. 8l 



tens osseus, respectively, and I confess that it is not obvious 

 why it should be necessary to go to the surface merely for 

 the purpose of getting rid of gas, presumably carbondioxide, 

 particularly since this gas is so readily soluble in water; 

 and I confess, also, that it is perfectly comprehensible why 

 it would be helpful to respiration for the animal to take in 

 atmospheric air occasionally, even though it depended 

 largely for its supply of oxygen on taking it from the water 

 by means of its pharyngeal movements. There is no doubt 

 that animals confined for a considerable time in a small 

 quantity of water do take air frequently, but their method 

 of doing this is quite different from the process described 

 above. A large part of the head is raised above the surface 

 of the water, the mouth is opened to a considerable width, 

 and a good quantity of air is gulped down. But I have 

 very rarely seen this done by animals in their native waters, 

 whereas the other operation may be witnessed as many times 

 as an observer might desire, where the animals are as 

 numerous as they usually are in the reservoir of which I 

 have previously spoken. But of course the various ques- 

 tions here raised concerning respiration can only be settled 

 by experiment and chemical examination. 



Gage ('90 and '91) has shown that in D. viridescens the 

 oral epithelium is columnar and ciliated while the animals 

 are strictly land-living, but is pavemented and non-ciliated 

 during aquatic life ; i. e., during the larval, branchiate period, 

 and when the aquatic habit is reverted to in later life by the 

 adults. 



While in general my observations on D. torosus confirm 

 Gage's results, I find some striking exceptions. 



On March 9th last, I examined the oral epithelium of 

 eleven large, smooth-skinned, wide-tailed males from the 

 reservoir, all of which I suppose had lived constantly in the 

 water for six months at least. At any rate, there was nothing 

 wanting in them of the characteristics that distinguish the 

 fully aquatic forms to lead me to suppose that some of them 

 had been water dwellers for a longer time than had the 

 others. In two of them, however, I found columnar cells 



