194 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



moistening the olfactory epithelium. Usually there is little complica- 

 tion of the olfactory surface, but in a few urodeles (Plethodori) there 

 is a projection from the lateral wall, the first indication of the conchae 

 which acquire such development in the higher groups. There is fre- 

 quently a differentiation of the nasal passage into a ventral respiratory 

 duct lined with ordinary and a more dorsal olfactory duct lined with 

 sensory epithelium. In the lower urodeles the diverticulum repre- 

 senting the organ of Jacobson is on the medial side of the nasal cavity; 

 a little higher it is ventral, while in the highest urodeles it has rotated 

 to the lateral side. It may be noted that some of the amphibia have 

 smooth muscles to close the external nares. 



Aside from the varying position of the choanae the changes from 

 amphibia to reptiles in the olfactory organs are comparatively slight. 



FIG. 198. Longitudinal section of nasal region of alligator, after Gegenbaur. c, concha; 

 ms, maxillary sinus; n, naris; p, pseudoconcha. 



The olfactory region becomes more distinct from the respiratory tract 

 and the latter shows a tendency to be differentiated into an anterior 

 atrium or vestibule, a middle area connected with the olfactory region, 

 and a posterior naso-pharyngeal duct between the basis cranii and 

 the roof of the mouth. This latter duct varies in length accordingly 

 as the choanae are anterior or posterior in position, the extreme being 

 reached in the crocodiles, where by ingrowth of palatines and ptery- 

 goids, the internal nares are carried back nearly to the hinder end of 

 the skull. A single concha, supported by bone, is developed in the 

 lateral wall of the reptilian nose. It is weak in the turtles (fig. 197), 

 but is larger elsewhere, and in the crocodiles (fig. 198) it becomes 

 divided in front, while a 'pseudoconch' (its homology with the supe- 

 rior concha of birds is uncertain) is developed above and behind the 

 true concha. Jacobson's organ occurs only in the squamata, where 

 it forms a simple pocket in the primitive position, ventral and medial 

 to the nasal cavity, near the nasal septum. 



