vii ACTION OF SENSORY STIMULI 335 



to which question we may refer to the fact that the stimulus 

 of light affects very primitive organisms, and that bacteria 

 travel towards the water that contains most oxygen. We can 

 also assert definitely that in many of the lower multicellular 

 animals simple epidermic, or at any rate tactile-cells, must be 

 capable of perceiving various kinds of stimuli, since touch and 

 taste-cells and .touch and olfactory- cells pass into one another 

 by gradual transition, and since touch-cells become in some 

 cases visual-cells, in others auditory ; while, on the other 

 hand, the so-called end-organs, which usually serve as organs 

 of taste, have in some cases become eyes, as Grenacher 1 has 

 shown in the Arthropoda. The evolution of organs of touch, 

 or of touch and taste into eyes is also beautifully shown in 

 leeches. It is obviously in the lower animals, whose senses 

 are not acute, more difficult to determine whether or how far 

 they perceive gustatory or olfactory stimuli, and still more, 

 whether they distinguish either from touch. But after 

 numerous researches and observations of my own, so much 

 seems to me certain, that originally one and the same cell 

 perceives different sensory stimuli, as, for instance, many 

 animals which have no eyes, such as insect larva? and worms, 

 are evidently affected by light through their epidermic cells, 

 and indeed it seems to act upon them as a painful tactile 

 stimulus. Probably all so-called sensory stimuli act upon 

 protoplasm originally in the form of tactile stimuli, hence 

 the fact that all sensory-cells first appear morphologically as 

 tactile-cells. Sensitiveness to diverse kinds of stimuli must 

 therefore have been gradually evolved in different ectoderm- 

 cells as a division of labour. 



Animals thus probably originally perceived light and 

 sound, and, to judge from anatomical relations, smell and 

 taste also, not as such, but as tactile sensation, and many 

 still so perceive them, by means of one and the same kind of 



1 Grenacher, Unlers. iiber das Sehorgan der Arthropoden, Gbttingen, 1879. 



