20 SMELL, TASTE, ALLIED SENSES 



the whole animal contracts. The fact that meat juice 

 will not excite the pedal edge of the column and that a 

 touch applied to the tentacles is seldom followed by more 

 than a slight local activity shows that the external surface 

 of the sea-anemone, though generally receptive, is locally 

 specialized. As a matter of fact this surface in degree of 

 dififerentiation stands between a diffuse receptive surface, 

 such as the vertebrate skin, and a speciahzed organ like 

 the eye or the ear. 



In the literal sense of the word the outer surface of a 

 sea-anemone is not sensory though abundantly receptive. 

 There is no reason to suppose that the receptive areas of 

 these animals are concerned with initiating impulses to 

 sensation. They connect very directly with muscles and 

 serve quite obviously as trigger-Uke organs by which the 

 muscle is set in action. A careful examination of the 

 activities of sea-anemones has failed to reveal any evi- 

 dence, such as can be produced from the more complex 

 animals, to show that these simple creatures possess 

 central nervous functions. Such functions apparently 

 have no part in their organized performances. Hence 

 their receptors have nothing whatever to do with initia- 

 ting impulses to sensation, but are limited in their action 

 to the excitation of the muscles after the type of the most 

 mechanical reflex. The presence in ooelentrates of eye 

 spots, olfactory pits, statocysts and other such special 

 receptors is, therefore, no indication that these animals are 

 endowed with corresponding sensations, as many of the 

 older workers believed, but this condition merely shows 

 that their possessors are especially open to a particular 

 stimulus. An eye spot does not mean that the animal pos- 

 sesses sight, but that it is readily excited to action by light. 



