456 



Comparative Animal Physiology 



The general chemical sense in vertebrates is obviously a development of 

 general irritability oF all mucous membranes in man and of the entire external 

 surface in aquatic animals. The receptors are probably free nerve endings. 



BEHAVIOR. In fishes olfaction is confined to the olfactory pits (only one 

 in the cyclostomes— monorhines), w/hich in some fishes, e.g., the hammerhead 

 shark, are widely separated. In sharks, rays, and dipnoans the nasal openings 

 are on the ventral side of the head, and in some cases the nasal pits also have 

 an opening into the mouth. The teleosts have dorsally located nasal pits 

 which do not open into the mouth, i.e., the water current is into the pit and 

 then back out (Fig. 148). The taste organs of fish are widely distributed. 

 They occur in the palatal organ of the mouth, the soft palate; in a ring 

 around the opening of the pneumatic duct (in some fishes) which leads to 

 the swim bladder; on the barbels; over the entire surface of the body; and on 

 the pectoral and dorsal fins.-"' Fishes locate food by means of gustatory 



Anterior opening 



\ Skin fold 



Posterior opening 



Skin fold 



Fig. 148. A, The head of a bony fish, showing the nasal pit. B, Longitudinal section 

 through the nasal pit, with arrows indicating the direction of water flow. 



and/or olfactory sense organs, and it is claimed that some fishes, if deprived 

 of their sense of taste, would starve to death in the presence of food. Cat- 

 fishes will snap at food or cotton soaked in meat juice held at their flank, but 

 will not do so if the nerve supplying the taste buds on the Hanks is cut. 



The amphibia have taste buds in the mouth and olfactory organs in the 

 nasal chambers which open both externally and into the mouth. The senses 

 of taste and smell seem to be at about the same stage of development in 

 amphibia as in fishes, but they are not nearly so important in feeding in am- 

 phibia as they are in fishes. The entire body surface, as in fishes, seems to 

 be quite sensitive to irritants, as seems to be true of all aquatic animals. 

 Among reptiles the sense of smell is not well developed, and the sense of 

 taste seems to be more highly developed in turtles than in snakes and alli- 

 gators. 



Birds, in general, have relatively poor chemical senses and depend largely 

 on vision for knowledge of their environment. The olfactory apparatus of 



