526 



COMPARATIVE ANATOMY. 



carried still farther, for a bridge of integument is drawn over the 

 pit, and the two separate openings are sometimes widely separated. 

 Both orifices — but most commonly the anterior ones — may project 

 forwards in the form of tubes. The investing mucous membrane 

 sometimes forms radial, sometimes parallel folds, by which its 

 surface may be considerably increased. The whole surface is supplied 

 with terminal branches of the olfactory nerve. There is another 

 kind of modification, in which the surface is so much increased 

 towards the exterior, by the extension of the olfactory mucous 

 membrane over a papilliform process, that the pit-like appearance 

 is altogether destroyed. 



In many Selachii, and in the Chimgerae the olfactory pit is 

 connected with the mouth, owing to the formation of a groove 

 (nasal groove), which passes from the pit to the angle of the 

 mouth (Fig. 292). The groove is frequently covered over by a 



median dermal fold, and is not un- 

 frequently converted into a deeper 

 canal (Eays). In this arrangement 

 we may observe a step towards 

 that which obtains in other Ver- 

 tebrata, where the olfactory pits 

 are only superficial in position 

 during an early period of em- 

 bryonic life. In them the arrange- 

 ment which is permanent in Fishes 

 disappears, and the processes which 

 go on during further development 

 cause the nasal pits to sink below 

 the surface. This is effected by the 

 great growth of the parts which limit 

 the pit in front, at the sides, and 

 in the middle line ; by the growth 

 of the edges of the groove towards one another a canal is developed 

 which leads from the pit to the primitive cavity of the mouth — 

 that is, from without, inwards ; this canal opens behind the margin 

 of the jaws. 



We find this arrangement in the Dipnoi and Amphibia. The 

 internal opening of the nasal canal is placed, in the former, as also 

 in the Perennibranchiata, just within the soft margin of the mouth. 

 In the Salamandrina and Anura it is bounded by firm parts of the 

 jaw-skeleton. 



When a nasal canal is formed, the primitive nasal pit itself is 

 removed from the surface. The surface of the pit then becomes com- 

 plicated by processes from the ethmoidal cartilage (turbiuate bones). 

 In the Amniota there are yet further modifications ; owing to these, 

 the upper part of the primitive buccal cavity is converted into a space 

 which takes in the nasal pit, and in its upper portion the olfactory 

 membrane is spread out. The primitive olfactory pit cannot, 

 therefore, be any longer distinguished as a separate organ ; and it 



Fig. 292. Inferior surface of the head 

 of Scy Ilium, m Mouth, o Entrance 

 to the nasal pit. n Nasal valve in its 

 natural position. n' Nasal valve 

 turned back, r Nasal groove. The 

 dots represent the orifices of the 

 mucous canals. 



