84 MENTAL EVOLUTION IN ANIMALS. 



with unmistakeable evidence of the occurrence of all the five 

 senses — or, more correctly, with unmistakeable evidence of a 

 power of adaptive response to all the five classes of stimuli 

 which respectively affect the five senses of man. 



Coming next to the Echinodermata, Professor Evvart and 

 myself have observed that Star-fish and Echini crawl towards 

 and remain in the light, even though this be of such feeble 

 intensity as scarcely to be perceptible to human eyes. 

 Moreover, we proved that this exceedingly delicate power of 

 discrimination between light and darkness is localized in the 

 pigmented ocelli situated at the tips of the rays in Star-fish, 

 and occupying the homologous positions in Echini. The 

 sense of touch we found likewise to be highly delicate, and 

 provided for by a variety of specially modified organs. 

 Lastly, I found that the sense of smell occurs in Star-fish, 

 though it is not localized in any special olfactory organs, 

 being in fact distributed equally over the whole of the 

 ventral surface of the animal, to the exclusion, however, of 

 the dorsal.* 



Among the Articulata we meet with numberless grades of 

 visual apparatus, from that of a simple ocellus, capable only 

 of distinguishing light from darkness, up to the greatly 

 elaborated compound eyes of insects and the higher Crus- 

 tacea. These compound eyes are remarkable from the fact 

 that each one of their possibly many thousand facets forms 

 an image of the corresponding portion of the visual field — 

 the multitude of separate sensory impressions being then 

 combined into a mosaic-like whole by a sensorial operation 

 taking place in the cephalic ganglion. In these compound 

 eyes, moreover, the images are thrown upon the receptive 

 nerve-surface without inversion. In the uncompounded or 

 simple type of eye, on the other hand, the image is inverted, 

 and as in the case of ants both kinds of eyes occur in the 

 same individual, it has been thought a psychological puzzle 

 how to explain the fact that mental confusion in the inter- 

 pretation of images does not result. A little thought, how- 

 ever, will show that the apparent puzzle is not a real one. 

 Thus it is commonly said that we ourselves really see 

 objects reversed, and that long practice enables us to correct 

 the erroneous impressions. But this statement of the case is 



* See Phil. Trans., 1881, Pt. Ill, Croonian Lecture ; and, for smell in 

 Star-fish, Journ. Linn. Soc, 1883. 



