86 MENTAL EVOLUTION IN ANIMALS. 



ternal epidermis invests the whole of the prominent eye-ball, 

 and forms in front of it a transparent horny layer, the 

 cornea." Fnrther, from the more recent observations of 

 Mr. Darwin, it is certain that Earthworms, although destitute 

 of eyes, are able to distinguish with much rapidity and pre- 

 cision between light and darkness ; and as he found that it is 

 only the anterior extremity of the animal which displays this 

 power, he concludes that the light affects the anterior ganglia 

 immediately, or without the intervention of a sense-organ.* 

 Lastly, Schneider says that Serpulse w411 suddenly Avithdraw 

 their expanded tufts when a shadow falls upon them ; but the 

 shadow must be that of an object moving with some rapidity. -|- 

 Turning now to the sense of hearing in the Articulata, we 

 find the simplest type of ear among the Vermes, where it 

 occurs as a closed globular vesicle containing fluid in which 

 there is suspended an otolith.J In some of the Crustacea, 

 such as the cray-fish and lobster, the organ of hearing is 

 much more complex, and here, " if we give rise, by playing 

 the violin, to notes of varying pitch, and at the same time 

 observe the auditory organ under the microscope, we see that 

 at each note only a particular auditory hair is set in vibra- 

 tion."§ Among Insects organs of hearing certainly occur, at 

 least in some species, although the experiments of Sir John 

 Lubbock appear to show that ants are deaf. The evidence 

 that some insects are able to hear is not only morphological, 

 but also physiological, because it is only on the supposition 

 that they do that the fact of stridulation and other sexual 

 sounds being made by certain insects can be explained ; and 

 Brunelli found that when he separated a female grasshopper 

 from the male by a distance of several metres, the male began 

 to stridulate in order to inform her of his position, upon 

 which the female approached him.|| I have myself published 

 observations proving the occurrence of a sense of hearing 

 among the Lepidoptera.^ Turning to the morphological side 

 of the subject, it is remarkable that in the Articulata the 



* See EartJiioorms^ pp. 19-45. f Ber thierisohe Wille, s. 194. 



X Earthworms have no ears and are totally deaf, although very sensitive 

 to vibrations communicated through contact with solid bodies. (See Darwin, 

 loc. cit., pp. 26-7.) 



§ Hseekel, loc. cit., English translation. International Library of Science 

 and Freethought, vol. vi, p. 325. 



II See Houzeau, Fac. Mem. des Animaux, t. i, p. 60. 



i See Nature, vol. xv, p. 177. 



