286 CHEMICAL SENSES 



food (crab) readily, provided that body juices from the bait diffuse into the 

 water, even when no visual cues are available as to the presence of the food. 

 A piece of cheesecloth soaked in crab body juice and attached to a stone was 

 repeatedly attacked. The animals approached all odorous preparations by 

 performing "searching" swim patterns, followed by circling movements 

 around the odor source. Animals that had been made anosmic by plugging 

 the nostrils with cotton wool ignored crabs placed in the experimental tank. 

 Removing the plugs restored the normal behavior pattern. Similar results 

 were obtained by Sheldon when nerve transection had produced anosmia. 



Further work by Parker and Sheldon (1913) did not add significantly to 

 the basic understanding of the problem. Mustelus canis was observed to 

 locate injured crabs more quickly than live ones. The presence in the tank of 

 crabs with flesh exposed elicited in nearby sharks rapid locomotor responses, 

 such as quick turning, and search movements over the bottom. "The head 

 was moved rapidly from side to side as the fish swam slowly, coursing, in 

 gradually diminishing circles, 2 or 3 inches from the bottom." When the 

 shark came within 5 to 8 cm, it seized the crab suddenly, shaking it violently 

 from side to side. Vision did not appear to play a role in the localization. 

 Sharks were often observed biting into the bottom of the tank at the exact 

 site where a crab had lain. These and earlier observations confirmed that 

 these sharks recognize and localize food through some chemical sense. Sub- 

 sequent experimentaion with anosmic animals confirmed the essential role of 

 olfaction in that behavior. 



MECHANISMS OF ORIENTATION THROUGH OLFACTION 



The important question of the mechanism of orientation through olfaction 

 was raised by Parker (1914), also in Mustelus. If odor in the water exerts "a 

 directive influence," he reasoned, then this influence should be affected if 

 the shark were rendered anosmic unilaterally. The direction of the move- 

 ments of normal and unilaterally anosmic animals, as well as the time re- 

 quired to locate the odor source of crab meat, were recorded. The time was 

 about the same in the two groups of fish, and the normal subjects made an 

 equal number of left and right turns. The experimental animals, which had 

 one nostril obstructed with a cotton plug, turned predominantly toward the 

 side of the functional nostril, making typical circus movements like those 

 described earlier in invertebrates with unilateral sensory dysfunction. Re- 

 moving the plug reestablished the normal locomotor pattern, consisting of an 

 equal number of left and right turns. [Parker held these results to demon- 

 strate that the shark, in approaching the source of an odor, turns con- 

 sistently in the direction of the nostril that receives the strongest olfactory 

 stimulation, localizing the food, therefore, by osmotropotaxi^J In a uni- 

 laterally anosmic animal the movements must become predominantly cir- 

 cular in pattern, although in sharks, unlike invertebrates, a number of move- 

 ments continue to be made in the direction of the nonfunctional nostril. 

 This results in figure-eight locomotor patterns. The noncircus movements 



