io8 



MORPHOLOGICAL TYPES 



spiracle of the dogfish, and each is closed laterally by the 

 ear-drum or tympanic membrane. These cavities are also 

 called tympanic cavity, and " middle-ear," and will be referred 

 to again in connexion with that sense-organ. The mouth 

 is provided with salivary glands, which assist digestion. 



The glottis leads to the larynx and lungs. The remaining 



Fig. 51. — Lacerta : ventral view of the pectoral girdle and sternum. 



c, coracoid ; cl, clavicle ; h, humerus ; ic, interclavicle ; r, ribs ; 

 s } sternum ; sc, scapula. 



viscera do not differ sufficiently in detail from those of Triton 

 to necessitate a specific redescription. 



Respiratory System. — The lungs are sacs with very vascular 

 walls, and they are the only respiratory organs, for the skin no 

 longer functions as such. Another change from the amphibian 

 condition is shown by the method of breathing. Instead of 

 raising and lowering the floor of the mouth, the ribs are pulled 

 forwards by muscles which run obliquely from rib to rib. At 

 rest, the ribs slope backwards, and when pulled forwards the 



