Dermal Sense- Organs of the Crustacea. 311 



III. The Piiysiologkial Import of the Dermal 

 Sense-organs. 



In discussing tlie pliysiological function of the dermal 

 sense-organs of Crustacea we must as far as possible guard 

 against antliroponior|)liic conceptions. It is advisable to define 

 the sensations by means of their ])hysieal or chemical causes. 

 The perception of an image originating in tlie eye we term 

 sight, the perce])tion of the waves of sound, hearing, while 

 the perception of the different kinds of resistance to pressure 

 and many other mechanical influences we call touch. In the 

 case of aquatic Crustacea it aj)pears to be a matter of choice 

 whether we speak of the perception of chemical substances 

 dissolved in the water as smell or taste. Crustacea possess 

 no sense-organs within the buccal cavity which, by virtue of 

 their position, Ave could explain as organs of taste, and those 

 sense-organs situated outside the buccal cavity (upon the 

 antenna?) which are adapted to the perception of chemical 

 substances dissolved in water may serve equally well for the 

 detection and taste of food-matter as for the perception of any 

 other stimulus depending upon chemical influence. I there- 

 fore see no reason, in the case of Crustacea which live in 

 water, for drawing a distinction between taste and smell. 

 We should exercise the o-reater caution in wishinsr to reco^-- 

 nize in Crustacea the same sensations which are experienced 

 by human beings, since the structure of the sense-organs is 

 fundamentally different in the two cases, while even the biolo- 

 gical purposes which the sense-organs serve can only coincide 

 to a limited degree. It is very possible that the Crustacea 

 possess senses entirely unknown to us, as, for instance, a 

 sensation which is affected by the amount of oxygen in the 

 water*. It is perfectly certain that the degree of acuteness 

 as well as the extent of the sensations, that is the limits 

 within which perceptions are possible for the various senses, 

 vary extraordinarily in different animals. The ej^e of a bird 

 of prey and the olfactory organ of a dog far surpass in acute- 

 ness of perception the respective sense-organs in the human 

 being. It is well known that many Insects perceive rays of 

 light and waves of sound which have no effect upon our owa 

 sense-organs. 



We will now discuss the question as to how far we may 

 draw conclusions from the morphological structure of the 

 sense-organs as to the physiological functions of the senses. 



* The Crustacea possess sense-orgaus the function of which is veiled 

 in obscurity, e. </. the frontal organ of the Entomostraca. 



