82 MENTAL EVOLUTION IN ANIMALS. 



response is made. It therefore seems certain that here we 

 have not to deal with what physiologists call the period of 

 latent stimulation, but with the time during^ which the lio^ht 

 requires to fall in order to constitute an adequate stimulus ; 

 just as a pliotographic plate requires a certain period of expo- 

 sure in order to admit of the luminous vibrations throwing 

 down the salt, so with the ganglionic material of this sense- 

 ori^jan. How different is the efficiency or development of 

 such a visual apparatus from that of a fully perfected retina, 

 which is able to effect the needful nervous changes in response 

 to a stimulus as instantaneous as that supplied by a flash of 

 lightning.* It is remarkable, looking to the Medusae as a 

 whole, in what a wonderful degree these primitive sense- 

 organs vary as to their minute structure in different species. 

 Nerve-cells and fibres, wrought up into more or less complex 

 forms, are clearly discernible in all those which have hitherto 

 been carefully examined; but when the particular specific 

 forms are compared with one another, it seems almost as if 

 organs of special sense, where they first undoubtedly occur 

 in the animal kingdom, revel, as it were, in the variety of 

 forms which they are able to present. 



It is probable, from the structure of the lithocysts, that 

 the Medusae are also affected by sonorous vibrations, and it 

 is certain that they are richly supplied with a variety of 

 organs ministering to the sense of touch. For not only are 

 they furnished with numerous long, highly sensitive, and 

 contractile tentacles, but in some species the marginal 

 ganglia are provided with minute hair-like appendages, 

 which must enable the nerve-cells to which they are attached 

 to be exceedingly sensitive to anything touching the hairs. 

 And, in connection with the sense of touch in the Medusae, I 

 may allude to my own observations on the precision with 

 which the point of contact of a foreign body is localized. 

 A Medusa being an umbrella-shaped animal, in which the 

 whole of the surface of the handle and the whole of the con- 

 cave surface of the umbrella is sensitive to all kinds of stimu- 

 lation, if any point in the last-named surface is gently 

 touched with a camel-hair brush or other soft (or hard) 



* For a full account of these experiments, see Phil. Trans., vol. 166, 

 Pt. I, Croonian Lecture, where it is shown that in otlier species of Medusae, 

 the sense-organs of which are more highly developed, there is no such pro- 

 longed delay in the response to luminous stimulation. 



