ANATOMY OF THE OLFACTORY ORGAN 37 



tory sac in the cyclostomes notwithstanding the fact that 

 this sac shows evidence in its deeper parts of being a 

 double organ. In consequence of single nasal openings 

 Amphioxus and the cyclostomes are commonly contrasted 

 with other fishes, and in fact with all other vertebrates, 

 and are called monorhine. Those in which the olfactory 

 organs are obviously paired have been designated as 

 amphirhine. 



In the sharks and rays the paired olfactory pits are 

 situated usually on the ventral side of the snout. (Fig. 11 ) . 

 The single opening of each pit is more or less divided by a 

 fold of skin into an anterior inlet and a posterior outlet 

 the latter sometimes leading into the mouth. As the fish 

 swims through the water and particularly as it takes 

 water into its mouth in breathing, a current of water is 

 passed through each of its olfactory sacs. In this way the 

 olfactory organs become associated with the respiratory 

 current, a condition that is more pronounced in the lung- 

 fishes than in the sharks and rays, for in the lung-fishes 

 the anterior apertures are external and form true anterior 

 nares, and the posterior openings lie within the mouth 

 and correspond to the choanse of higher vertebrates. In 

 the highly speciahzed bony fishes, the paired olfactory pits 

 are almost always on the dorsal aspect of the head and 

 quite distant from the mouth. Each pit has two entirely 

 separate openings, an anterior inlet and a posterior outlet. 

 By means of these two openings a current of water enters 

 and leaves each pit. This current is produced either by 

 ciliary action within the pit (Amiurus) or by the action of 

 the muscles associated with the jaws and gills (Fundulus) . 

 In bony fishes, then, the olfactory pits are purely recep- 

 tive and are in no direct way connected with the respira- 



