526 



UKODELA 



[CH. 



jaw will be seen to throb at regular intervals, being alternately 

 puffed out and drawn in. It can be further seen that the nostrils 

 are closed when the skin is drawn in and opened when it is puffed 

 out. These movements constitute the mechanism of breathing in 

 the newt. As in the case of the Dipnoi, the paired nasal sacs 

 communicate with the interior of the mouth by openings called 



Fio. 254. Diagram illustrating three stages in the development of the vertebral 

 column of an Opisthocoelous Urodela. 



A. Stage in which basidorsals, basiventrals and intercalaries are separate. 



B. Stage in which basidorsals and basiventrals have united with one another 

 and in which the intercalaries have united to form an intervertebral pad of 

 cartilage in which a synovial cavity is just appearing. This stage is per- 

 manent in some Urodela. 



0. Stage in which the vertebrae are complete. 



1. Basidorsal. 2. Notochord. 3. Basiventral. 4. Dorsal inter- 

 calary. 5. Ventral intercalary. 6. Synovial cavity. Dotting, 

 cross-hatching, etc. as in Figs. 232 and 242. 



the choanae or internal nares, and the air passes through these 

 from the nostril when the cavity of the mouth is enlarged. When 

 the cavity of the mouth is compressed the nostril is closed by a flap 

 of skin constituting a valve, and the air is forced through the open 

 glottis into the lung, whence it is forced out again by the elastic 

 recoil when the pressure is removed. 



