144 ONCHIDIUM. 



different development, arising, except the nerve, entirely 

 from the integument ; on the contrary, in the vertebrate 

 eye, while the cornea and lens are formed from the 

 skin, the retina is an outgrowth from the brain. 



Semper does not suppose that the Onchidia perceive 

 any actual image with their dorsal eyes, and thinks that 

 they are merely able to distinguish differences in the 

 amount of light. 



They are shore-living molluscs, and are preyed on 

 by small fishes belonging to the genus Perophthalmus, 

 which has the curious habit of leaving the water and 

 walking about on the sand in search of food. The 

 back of the Onchidium contains a number of glands, 

 each opening by a minute pore ; and Semper suggests 

 that, when warned by the shadow of the fish, the little 

 slugs eject a shower of spray, drive off their enemy, 

 and save themselves. This is not quite so far-fetched 

 as might at first sight appear, for we know that there 

 are many other animals, the sepia, many ants, the 

 bombardier and other beetles, etc., which defend them- 

 selves in a similar manner. 



It seems difficult to understand why the Onchidia 

 should be endowed with so many eyes. The irrelative 

 repetition of organs meets us, however, continually in 

 the lower animals. Moreover, in the present case 

 Semper has thrown out a plausible suggestion. The 

 organs of touch (see ante, p. 14) curiously resemble 

 eyes in structure, and a very slight change might 

 make them capable of perceiving light. It is possible, 

 then, that some of them may undergo a change of 

 function, and that this may throw some light on the 

 variability in number. 



In the ChitonidsB, where dorsal eyes have recently 



