292 ORGANS OF THE SENSES. 



body. By the separation of the organs of smell from the 

 respiratory passages in fishes, as in other water-breathing 

 animals, their great sensibility and delicate structure are pro- 

 tected from the violent and incessant action of the currents 

 of water required for respiration. In most osseous fishes there 

 is an anterior contractile and a posterior open or valvular 

 orifice placed superficially apart from each other, and leading 

 into each nostril from the upper part of the muzzle, but in 

 the lampreys both nostrils open by a common orifice on the 

 upper part of the head. In the plagiostome species the 

 wide plicated valvular nostrils open on the under surface 

 of the face anterior to the mouth, and in some of the eels 

 an approach is made to the structure of these parts in 

 the lowest amphibia by having the posterior orifice of their 

 nostril placed internally under the upper lip. 



In the amphibious animals, where the respiration of air 

 begins to be effected through the nostrils, the olfactory 

 organs become more complicated in structure and more 

 internal in their position. The cartilaginous plicated portion 

 of the organ of fishes now begins to assume the more 

 compact tubular and convoluted form which the osseous 

 plates present in the higher classes of animals, and the sen- 

 sitive surface of the organs increases in extent as we ascend 

 through the vertebrated classes. In the perennibranchiate 

 amphibia the nostrils still form on each side a simple sac, 

 without internal convolutions, and having their posterior 

 opening so far forward in the mouth as to be immediately 

 under the upper lip, as in some fishes. In the larva state 

 of salamanders and frogs the nostrils are still, as in fishes, 

 confined to the exterior of the head, and even in the adult 

 forms of these animals the posterior openings of the nostrils, 

 though within the cavity of the mouth, are much advanced 

 in their position, and distant from the median line of the 

 body. The exterior nares which are muscular and contrac- 

 tile, have now almost lost the cartilaginous valve of the fishes, 

 but the turbinated bones even in a cartilaginous form, scarcely 

 yet extend the interior surface of the organ, and the cavities 

 of the nostrils in the proteus are still laminated as in a fish, 

 although they open posteriorly under the upper lip. 



In the serpents the organs of smell have their internal 

 surface extended by the rudimentary turbinated bones, and 



