494 



Comparative Morphology of Chordates 



Trachea 

 Right carotid artery 



Right subclavian 



artery Wj 



Esophagus 

 % — Left jugular vein 

 ]W) Left subclavian 



vein 

 Lung 



Stomach 





Accessory' 

 bladder 



Urinary bladder 



VISCERA OF FEMALE TORTOISE 

 ( Emys europoea) 



Fig. 386. Turtle viscera: ventral view. Most of the stomach is hidden by the 

 left lobe of the liver. (Modified from Owen: " Comparative Anatomy and Physiology 

 of Vertebrates," London, Longmans, Green & Co., Ltd.) 



of lateral "accessory bladders" of uncertain function (Fig. 386). All 

 chelonians are oviparous. 



The most distinctive features of Chelonia are those which relate 

 to the shell. It is composed mainly of dermal bone. The living epi- 

 dermis is entirely external to the bony plates and produces the super- 

 ficial horny plates. The trunk vertebrae, usually 10, have massive 

 neural processes which expand dorsalward and contribute a consider- 

 able amount of bone to the overlying dermal neural plates, with 

 which they are fused (Fig. 385). Similarly, each rib of the trunk ex- 

 pands into a broad, flat plate which fuses with, and merges indistin- 

 guishably into, the dermal bone of a costal plate. The marginal and 

 other plates of the carapace and the plates (usually four pairs and 

 one median) of the plastron are wholly dermal (Fig. 385). In the 

 "soft -shelled tortoises" the horny layer does not form definite scales, 



