282 



AMERICAN TESTUDINATA. 



Part II. 



The trachea is generally rather short, divided near the base of the neck into 

 two large bronchi, one of which is often so curved as to form a large arch. The 

 lungs are very voluminous ; more so in land than in water Turtles. This differ- 

 ence alone, in the size of this organ, accounts almost entirely, both for the high 

 arched body of the true land Turtles which never go into water, and for the 

 flat trunk of the Trionychidse and sea Turtles, which hardly ever leave the water, 

 except to lay their eggs. But even in the aquatic Turtles, the capacity of expan- 

 sion of the lungs is great enough to allow them to remain for half an hour or 

 more under the water, as I have had ample opportunities of observing in Trionyx, 

 though it must not be forgotten, that in the family of Trionychidse, the skin being 

 soft and thus more permeable to water, a kind of respiration of the blood may 

 take place through the skin also,^ as is the case so extensively in Frogs. 



The following table shows the capacity of the lungs in those families, of which 

 I was able to obtain fresh specimens at the time. The experiments were made 

 upon the living animal by pumping out the air of the lungs, then pumping in 

 water, then pumping out the water again and measuring its amount in cubic 

 inches. This table shows that aquatic Turtles require much less air in their lungs, 

 in proportion to the weight of the body, than land Turtles.^ It shows especially, 

 that in mud and soft-shelled Turtles, the lungs being much reduced in size and 

 importance, by far the greater part of the respiration must be performed by the 

 skin of the whole body, wliich is much thinner in these families than in other 

 Turtles ; while, on the contrary, in the true land Turtles and that land Emydian, 



backwards. Again, we find in Frogs, at least in 

 some, for instance in the genus Rana, a movable valve, 

 by whieli it can close or open the nostrils at will ; 

 there is nothing of this kind to be found in Turtles. 



' The beautifully ramified vessels, which are seen 

 through the epidermis upon the entire lower sur- 

 face of the body of Trionyx, add great weight to 

 this supposition. See below, p. 284. 



^ It is moreover evident that the capacity of 

 the lungs is not a family character, for while the 

 Testudinina (land Turtles) are generally provided 

 with much larger lungs than the Emydoidre, our 

 table furnishes the unexpected evidence, that in a 

 member of the latter family they are larger still. 

 The capacity of the lungs in Cistudo, for instance, 

 shows clearly its influence upon the form of the 

 body, and it would thus seem that here, at least, 

 form cannot characterize the family. But this very 

 instance proves, on the contrary, the truth of the 



principle adopted for the limitation of families, as 

 by a thorough examination we find still in the 

 Cistudo the real character of the form of Emyds, 

 in its sharp contradistinction from the Testudo 

 family. See below, The Family Characters of Emyd- 

 oidiE. Hence it follows, that the mode of life, and, 

 what depends upon it in the organization of the 

 animal, the capacity of the lungs, the length and pro- 

 portions of the intestine, etc., are generally, though 

 by no means always, common to a family ; and that 

 such definite complications of forms as characterize 

 families may be modified according to the diflerent 

 modes of life, without interfering with or changing 

 the ideal combination. This ideal is the conception 

 of the Divine Mind. The conception however is 

 not changed, in the act of being expressed in living 

 realities, but only specified ; and this is done in the 

 various members of a family, according to their 

 mode of life, etc. 



