THE YPSILOID APPARATUS OF URODELES. 28l 



will be referred to almost exclusively in the following discussion. 



As has been said, the adult Dicinyclylus is the most aquatic of 

 our American salamanders. To anyone who has observed, even 

 casually, the activities of this little animal in the water, its abso- 

 lute ease under aquatic conditions must have been evident. If 

 the specimens are in a deep aquarium this physical ease is very 

 readily observed. Occasionally they may be seen at the bottom 

 where they walk about or take rapid little swims to higher levels 

 from which, as soon as the swimming motions cease, they pas- 

 sively sink to the bottom again. More often they may be found 

 at the very surface of the water where they float with great ease 

 or rest upon the aquatic plants, sometimes supporting themselves 

 upon these by the fore limbs and lifting the entire head above 

 the water. Frequently, moreover, they will be seen suspended 

 in the water at a greater or less depth, where they have the power 

 to swim lazily to and fro with hardly a perceptible muscular 

 action, to paddle about, using all four feet as propellers, or to 

 dart swiftly through the water by means of a rapid lashing with 

 body and tail, the legs meanwhile being closely pressed to the 

 sides of the body. For many minutes they will sometimes re- 

 main absolutely motionless in the water, the body kept in place 

 by the mere contact of a foot, or even of a single toe, with some 

 plant or other stationary object. 



These facts indicate that while the specific gravity of Dicniyc- 

 tylus is never far from one, it varies slightly, as shown by the 

 passive sinking, suspension or floating of the body at different 

 times. What are, then, the mechanical means by which these 

 changes in buoyancy are accomplished ? 



If our observations begin with a Diemyctylits at the bottom of 

 the aquarium, with a specific gravity greater than one, it will be 

 found that sooner or later the animal will swim rapidly to the sur- 

 face, and, by the modified process of pulmonary respiration already 

 described, will take into the lung sufficient air to cause the body 

 to float (Fig. 10, b\ the minute portion of the back which appears 

 above the surface bearing witness to the fact that the specific 

 gravity has now become slightly less than one. The animal 

 may remain in this condition for a longer or shorter time. If he 

 swims to a lower level, the moment his motions cease his body 



