Sensory Discrimination: The Chemical Sense 69 



of discrimination is chemical. Lacrymaria, another ciliate, 

 tests with its 'head' "every object within reach and rejects 

 all those which cannot serve as food. It does not swallow 

 inorganic substances, carmine, or ink particles and the 

 like. This protozoon unquestionably exercises selection in 

 feeding" (469, p. 243), but the basis of the selection is not 

 determined. Didinium is a ciliate which has a peculiarly 

 modified seizing organ, but the only selection of food which 

 it makes rests on the fact that this organ will adhere 

 to the surface of some organisms and not to that of others 

 (466). Two other protozoa, Actinobolus radians and 

 Spathidium spathula, have each so far refined the process 

 of selection of food that they swallow only one kind of 

 organism. Actinobolus, an anchored form, awaits its 

 destined prey, and Spathidium selects it in freely swimming 

 about ; but as to whether the prey is recognized by chemical 

 or by mechanical features we have no information (499). 



1 6. The Chemical Sense in Ccelenterates 



The lowest of the Metazoa, or many-celled animals, 

 are the ccelenterates. Although externally the forms of 

 different families of ccelenterates differ widely, yet the 

 general plan of structure is the same in all : the body of 

 the typical ccelenterate is a hollow sac, whose walls con- 

 sist of two layers of cells, food being taken into a mouth 

 at one end of the sac, and the arrangement of cells being 

 on the plan of circular symmetry. In the phylum of the 

 ccelenterates are included sea-anemones, jellyfish, the little 

 green or yellow Hydra, sponges, corals, and ctenophores. 



Hydra (Fig. 6), one of the simplest ccelenterates, shows 

 a food reaction distinct from the contact reaction. Me- 

 chanical stimulation is followed by withdrawal of the ten- 



