Sensory Discrimination: The Chemical Sense 75 



authorities disagree, and probably species differ. Nagel 

 finds the tentacles most sensitive (521) ; Loeb observed 

 that the stump of the animal has discriminative reactions 

 (427), while Fleure and Walton state that in the species 

 tested by them the mouth-region is most responsive to 

 chemical stimulation (228). 



A certain amount of discrimination between mechanical 

 stimuli is ascribed to these animals by Romanes. "I 

 have observed," he says, "that if a sea-anemone is placed 

 in an aquarium tank and allowed to fasten upon one side 

 of the tank near the surface of the water, and if a jet of 

 sea water is made to play continuously and forcibly upon 

 the anemone from above, the result of course is that the 

 animal becomes surrounded with a turmoil of water and 

 air bubbles. Yet after a short time it becomes so accus- 

 tomed to this turmoil that it will expand its tentacles in 

 search of food, just as it does when placed in calm water. 

 If now one of the expanded tentacles is gently touched 

 with a solid body, all the others close around that body in 

 just the same way as they would were they expanded in 

 calm water" (642, p. 48), although the solid stimulus is 

 decidedly less intense than that offered by the bubbles. 

 Similarly, Fleure and Walton find that certain species 

 show little reaction to accidental contact with a pebble 

 that is moved, but react quickly to a finger (228). 



The body of a typical medusa or jellyfish consists of a 

 bell-shaped "umbrella" from the edge of which tentacles 

 depend. Hanging from the middle like the clapper of the 

 bell or the handle of the umbrella is the manubrium, at 

 the end of which is the mouth. In the medusa Carmarina 

 hastata no differentiation in reaction to contact and food 

 stimulation appears, merely a readier response of the ten- 

 tacles to the latter ; but we do find whatever evidence for 



