Sauropsida: Class Reptilia 



495 



and the underlying dermal plates are incompletely ossified. In the 

 marine "leatherback" turtle (Dermochelys or Sphargis) the cara- 

 pace is not joined to the endoskeleton, the dermal plates of the shell 

 are arranged in 12 longitudinal rows, and the horny layer is not formed 

 into scales. 



A most perplexing peculiarity of the chelonian is the relation of the 

 pectoral girdle to the ribs. In 

 other animals the girdle is ex- 

 ternal to the ribs. Thus the hu- 

 man "shoulder blade" (scapula) 

 is the outermost skeletal part in 

 the upper region of the back. The 

 chelonian pectoral girdle is com- 

 pletely inside the ribs. The long 

 scapula is movably attached dor- 

 sally to the inside of the carapace 

 (Fig. 385). 



Fusion of the carapace with 

 the vertebrae of the trunk elimi- 

 nates possibility of relative motion 

 among those parts. In the absence 

 of anything that can be moved, 

 there is a corresponding absence 

 of muscles. Therefore the muscles 

 of the body are reduced to those 

 which have to do with movement 

 of the legs, neck, and tail. 



Enclosure of the body within 

 the shell and the immobility of the 

 ribs make it impossible for the 

 chelonian to breathe as other rep- 

 tiles do. Breathing is accom- 

 plished partly by the amphibian 

 method of "swallowing" air and 

 partly by varying the external 

 pressure on the lungs, which are a pair of very capacious and internally 

 subdivided sacs lying in the anterior part of the pleuroperitoneal cavity 

 (Fig. 387). The pressure is varied by alternately protruding and retract- 

 ing the head and neck. In retraction the neck is strongly pushed back 

 against the anterior body-wall in the space between carapace and 

 plastron. The pressure on the lungs may be made to vary also by move- 

 ments of the muscles of the legs. Animals of the notoriously sluggish 

 habits of chelonians are probably not seriously inconvenienced if the 

 breathing is somewhat desultory. 



Fig. 387. Lung of a large turtle 

 (species?). Length of lung about 7 

 inches. Photographed from a dried lung 

 cut in a frontal plane. 



The large chambers open freely into 

 a central air-passage. Into them open 

 smaller cavities whose walls are thickly 

 beset with shallow alveoli which add 

 to the respiratory surface. (B) Bronchus. 



