vn SEiVSE- CELLS OF A URELLA 



333 



cells is, as I have shown, seen in a wonderfully beautiful 

 way in the sense-organs of the toponeurous Medusae, e.g. 

 of Aurelia aurita.^ Here at particular spots in the margin 

 of the bell, on the so-called marginal bodies and in their 

 neighbourhood, lie in close proximity, visual, auditory, and 

 probably olfactory organs, as well as tactile organs, formed 

 by groups of epidermic-cells specifically modified. A pave- 

 ment epithelium covers the greater part of the surface of the 

 bell. The cells of this epithelium at the edge of the marginal 

 bodies change first into flagellate cylindrical cells, which also 

 line certain depressions of the surface near the marginal 

 bodies, and here probably are subservient to smell or taste, 

 but also, like the ordinary flat epithelium, are doubtless 

 tactile. Such flagellate cells are modified here and there 

 into a kind of visual rod, and, the cells surrounding these 

 acquiring pigment, the eye of the jelly-fish is formed. Close 

 to these eyes on the clavate extremity of the marginal body 

 the epithelium again becomes flat, and a vesicle containing 

 crystals being developed beneath it, forms the auditory 

 epithelium. 



The history of the sensory-cells, their origin from one 

 and the same primitive form, even if nothing else did, woidd 

 indicate that all sensory stimuli may be but different 

 qualities of one and the same stimulus, difterent forms of 

 motion of external media — even the stimuli of taste and 

 smell. 



The coarsest of these stimuli is that of touch. To this 

 the epidermis-cells were in any case susceptible from the 

 beginning, and the protoplasm of even the lowest protozoan 

 is also affected by it. Whether or how far this is also true 

 of the other stimuli is not yet established. The remarkable 



1 Cf. Die Medusen, and my address to the Congress of Naturalists at Munich 

 in 1877: "On Artificial Divisibility and the Nervous System of the Medusa-," 

 published in the Proceedings of the Congress, also i^riuted Avitli additions in the 

 Arch./, mik. Anat. Bd. xv. 1877. 



