OLFACTOEY OECANS OF VEETEBEATA. 525 



§ 395. 



As it is more and more impossible to form auy opinion abont the 

 sense of taste^ the farther the organism examined is removed 

 from Man, it is impossible to speak with much certainty about the 

 gustatory organs of most of the Vertebrata. We can only there- 

 fore regard the end-organs of nerves, which are placed in the 

 mucous membrane of the mouth, as belonging in a general way 

 to this apparatus. In Fishes, these end-organs have no specific 

 character; they resemble rather the goblet-shaped organs which 

 are scattered in the external integument ; this is easily understood 

 when we reflect on what is the origin of the buccal cavity. Those 

 which are found in the palatine region are the most exactly known 

 (cf. Fig. 291) ; in the Cyprinoids the mucous membrane of this 

 region is interwoven with a large number of muscular fibres. In 

 the Amphibia the tongue appears to be the principal seat of these 

 structures, which are also known as "gustatory gohlet-cells;" 

 although these cells ai'e not ordinarily found on the tongue of 

 Reptiles and Birds, the tongue of the Mammalia is provided with 

 them, and they are placed on the sides of the papillaj cir cum vallate. 



Olfactory Organs. 

 § 396. 



In all Vertebrata the olfactory organs appear as shallow pits, 

 placed on the head, and enabled to receive excitations from the 

 surrounding medium by means of the rod-shaped end-organs of the 

 olfactory nerve. The sensory organ is therefore represented by a 

 differentiated portion of the integument. Although we cannot 

 certainly say that these structures have in aquatic forms — Pisces and 

 Amphibia — the same function as they certainly have in the air- 

 breathing forms, yet we are perfectly justified in giving them the 

 same name at any rate, for we see that they pass, and that in a 

 continuous series, into the more complicated organs of the higher 

 Vertebrata, which undoubtedly do serve as olfactory organs. 



In the Leptocardii the olfactoxy pit is unpaired, as it is also in 

 the Cyclostomata, where, however, it is converted into a deeper tube 

 (Fig. 239, (j'), which ends blindly in Petromyzon (gr), but in the 

 Myxionoidea has the form of a canal which passes through the palate ; 

 the walls of this canal are supported by a tube of cartilaginous rings. 

 In the Gnathostomata there are paired olfactory pits. In Fishes 

 they remain mere pits, or are but slightly deepened. In the Selachii 

 two processes project from the margin of a nasal orifice towards 

 one another, and divide the primitively simple orifice into an afferent 

 and an efferent orifice. In the Osseous Fishes this arrangement is 



