200 



COMPARATIVE ANATOMY 



except that, as already mentioned, 2^osterior (internal) as well as 

 anterior (external) nostrils are present. The latter open beneath 

 the upper lip, and so cannot be seen when the mouth is closed ; 

 the former open into the oral cavity rather further back. 



The peculiar position of the anterior nares has a physiological significance, 

 at any rate in Protopterus, in connection with its habits (see p. 17) ; during- 

 its summer sleep the animal breathes through a tube, passing between the 

 lips, formed from the capsule or cocoon which encloses it. The necessary 

 moisture for the olfactory mucous membrane during this time is provided; 

 by the numerous goblet cells which line the walls of both nostrils. 



Amphibia. — The olfactory organ of the Perennibranchiata re- 

 sembles in many respects that of the Dipnoi : it is always enclosed 



within a complete or perforated 

 cartilaginous capsule situated 

 laterally to the snout close be- 

 neath the skin, and is not pro- 

 tected by the bones of the skull 

 (Fig. 159). Its floor is largely 

 fibrous, and the mucous mem- 

 brane is raised into radial folds 

 like those of Cyclostomes and 

 Polypterus. In all the other 

 Amphibia it becomes included 

 within the cranial skeleton, and 

 lies directly in the longitudinal 

 axis of the skull ia front of the 

 cranial cavity. 



The structure of the olfac- 

 tory organ now becomes modified 

 in corre-spondence with the change- 

 in the mode of respiration; the- 

 nasal chamber becomes differ- 

 entiated into an olfactory and a 

 respiratory ijortion, and an ex- 

 tension of the olfactory surface takes place by the formation, 

 of one or more prominences on the floor and side-walls of 

 the nasal cavity. These prominences, which may be compared 

 to the turiinals of higher forms, arc present in certain Mycto- 

 dera (Fig. 160), and attain a very considerable development in 

 Anura and Gymnophiona, especially in the latter, in which the 

 nasal chamber is converted into a complicated system of spaces 

 and cavities. A main chamber and a more laterally situated' 

 accessor!/ cavity can in all cases be distinguished, even in the 

 Derotremata and Myctodera ; the accessory cavity lies in the- 

 maxillary bone (Fig. 160 and 165 A — E). 



In certain Gymnophiona the accessory chamber becomes entirely shut off" 

 from the main c.n'ity and reoei-^-es a special branch of the olfactory nerve,, 

 so that in these cases two separate nasal cavities can be distinsuished. 



Fig. 159. — Olfactory 

 Kerturus maculattis. 

 dorsal side. ) 



Okgak 

 (From 



the 



X, olfactory sac ; 01, olfactory nerve ; 

 Pmz, premaxilla ; F, frontal ; P, 

 process of the parietal ; PP, palato- 

 pterj'goid ; AF, antorbital process. 



