282 INEZ L. WH1PPLE. 



rises slowly to the surface again. Usually, however, the increase 

 of buoyancy is soon followed (within a minute or two) by the 

 emission from the lungs through the mouth of one tiny bubble of 

 air after another, seldom more than two or three in all, until his 

 buoyancy is so perfectly adjusted that his specific gravity is ex- 

 actly one. In this condition he can go about at ease, or remain 

 motionless at any depth, and it is apparently only when he de- 

 sires to sink to the bottom and remain there with some stability 

 that by the emission of still more air the specific gravity is made 

 sufficiently great to serve the purpose. There always occurs, 

 however, a gradual loss of buoyancy even when there is no 

 further emission of air, a loss which I have never observed to be 

 made good until the animal again swims to the surface and takes 



in more air. 1 



Diemyctylus shows a still further delicacy of adjustment to its 

 aquatic environment, since, under any condition, whether floating, 

 suspended in the water or resting on the bottom, there is the 

 power to change, without the slightest swimming motion, the 

 direction of the long axis of the body. This adjustment may be 

 best observed when the animal is suspended motionless in the 

 water ; since then all other factors which produce change of po- 

 sition are eliminated. The usual position of Diemyctylus when 

 thus suspended is one in which the anterior end of the body 

 slants slightly downward (Fig. 10, b\ From this position the 

 whole body, without the slightest bending, may swing through a 

 vertical angle of perhaps 30 until the head is directed upward 

 instead of downward. This change of direction is accompanied 

 by a striking change in 'the shape of the animal. When the 

 poise is such that the head slants downward, there is a pro- 

 nounced bulging of the lateral and ventral walls of the posterior 

 third of the body cavity, particularly noticeable in the angle be- 

 tween the ilium and the vertebral column, as though some 

 mechanism within were exerting an outward pressure. As the 



1 This phenomenon of loss of buoyancy without the emission of air is worthy of 

 careful investigation. It is probably not attributable to a mere compression of air (as 

 in the case of the air-bladder of the fish), since there is no subsequent increase of 

 buoyancy without taking in more air. It must be due to an actual loss of gas from 

 the lungs, probably owing to the excess of the volume of oxygen used over that of 

 CO 2 and other gases given back to the lungs. 



