PHYSIOLOGY OF SYNAPTULA HYDPJFORMIS 375 



tive type. In support of the former view, it may be urged that 

 not only is the histological structure of the vertebrate olfactory 

 sense cell the same as that of the invertebrate bristle cell, but 

 the physiological characters of the two kinds of cells are sim- 

 ilar. (1) The nature of the stimulus is the same, viz., chemical 

 substances, (2) they are 'distance receptors,' 1 and (3) a very 

 minute amount of substance serves as a stimulus. 



Nagel ('94) has advanced the idea of the existence of 'uni- 

 versal sense organs' in the invertebrates, i.e., organs which, 

 instead of being capable of receiving stimuli of only a limited 

 range, can receive stimuli of all sorts. In the course of evolu- 

 tion from the lower to the higher forms, certain of these 'uni- 

 versal sense organs,' according to this view, became more restrict- 

 ed in their sensitivity and responded to fewer and fewer classes 

 of stimuli, until finally such specialization occurred as is shown 

 in the present condition of man, where there are no recognizably 

 'universal sense organs' but some twenty (Herrick, '15, p. 74) 

 special senses. 



It may be that Synaptula affords an example of an animal 

 in which these universal sense organs exist. The methods of 

 determining whether or not sense organs are specialized to re- 

 ceive one class of stimuli only are (1) to apply these stimuli on 

 separate portions of a given area of body surface, and (2) to an- 

 aesthetize, if possible, a surface differentially. The latter 

 method seems to me untrustworthy when used alone. A 'uni- 

 versal sense organ' when slightly anaesthetized might not re- 

 spond to light, but might respond to other more vigorous kinds 

 of stimuli, such as the application of an acid. If, however, one 

 finds, in addition to differential anaesthesia, the existence of 

 definite regions which respond, e.g., to touch alone, or chemical 

 agents alone, then one must conclude that special sense organs 

 are present. The same conclusion is drawn if one finds that the 

 histological structure of the sense organs in one region differs 



1 In strict analysis 'distance receptor' is a misnomer, for organs to which this 

 is applied depend, like all other sensory organs, on actual contact with the stimu- 

 lating agent. It is therefore probable that '(2)' and '(3)' are only different ways 

 of expressing the same thing. 



