170 



COMPARATIVE ANATOMY. 



OLFACTORY ORGAN. 



The olfactory organ, in its simplest form, consists of a paired, 

 pit-like depression of the integument above the cleft of the mouth. 

 A nerve passes to the base of this pit, and becoming enlarged 

 in the form of a ganglion sends off nerves to the sensory cells 

 (olfactory cells). The latter must be regarded as phylo- 

 genetic derivatives of the end-bulbs of that part of the integu- 

 ment which becomes pushed in to form the primitive olfactory 

 pit ("olfactory bulbs"), and therefore come under the same 

 morphological category as the taste-bulbs. At first (that is in 

 Fishes and Urodeles), they are only separated from one another by 

 interstitial epithelial tissue, but from the Anura onwards this tissue 

 gradually disappears in order to allow of an increased surface for 

 the olfactory epithelium. The ciliated cells lying amongst the 

 sensory cells serve to continually renew the outer medium whether 

 that be water or air by which the odoriferous particles are conveyed 

 (Fig. 140, 32, M 



FIG. 140. EPITHELIUM OF THE OLFACTORY Mucous MEMBRANE. 

 Petromyzon planeri ; B, of Salamandra atra. 



E, olfactory cells ; E, interstitial epithelial cells. 



A, of 



While the olfactory organs of all the true Fishes exhibit the 

 above-described simple sac-like form, from the Dipnoi onwards 

 they come to communicate with the cavity of the mouth as well 

 as with the exterior. In consequence of this, anterior or external, 

 and posterior or internal nostrils (choanse) can be distin- 

 guished, and as a free passage is thus formed through which the 

 external medium can pass, the olfactory organ takes on an important 

 relation to the respiratory apparatus. 



