591.185 173 



535.7 



III. 



The Light-Sensations of Eyeless Animals. 



THE fact that certain skins are sensitive to light has long been 

 known. Most of us are familiar with the case of the earthworms, 

 whose sudden withdrawal into their holes on the approach of a lighted 

 candle was observed and described by Darwin. The earthworms 

 have no eyes, and there is no other apparent explanation of this 

 reaction but that the skin itself is sensitive to the light-rays. It is 

 also well known that this is not the case with all skins — our own, for 

 instance — and that, in the vast majority of animals, there is only one 

 organ, viz., the eye, which is sensitive to variations of light intensity. 

 The assumption is justifiable that if we could only explain the 

 mechanism of this diffuse light-sensation of the skin, usually called, 

 not very happily, the " dermatoptic " function, we should be in a fair 

 way towards understanding the mechanism of the exquisite sensitive- 

 ness of our eyes. It is, indeed, open to anyone to assume that this 

 diffuse sensitiveness was once universally present, but was lost on 

 the development of eyes, these specialised organs rendering the less 

 efficient function unnecessary ; and that eyes first arose merely as 

 localised areas, in which this sensitiveness to light was specially 

 concentrated. 



It is quite possible that this may have been the case: it is a fair 

 hypothesis ; but, on the other hand, the facts seem to show that the 

 dermatoptic function is not necessarily a primitive condition retained, 

 but may also be secondarily acquired. It is found, for instance, in 

 isolated members of different animal groups, and nearly always in 

 adaptation to some special manner of life ; this favours the idea that 

 it has been secondarily developed. The ancestors of the earthworm, 

 for instance, were probably more free-living chgetopods, and, as such, 

 almost certainly possessed eyes, which were lost in the earthworms 

 owing to their burrowing manner of life. I know of no record of any 

 modern chsetopod possessing the dermatoptic function. 



With regard to the snails, which possess both eyes and, to a 

 slight degree, the diffuse sense, shrinking at a passing shadow even 

 when the eye-bearing tentacles are cut off, we may, on first thoughts, 

 suppose the diffuse function either to have persisted along with the 

 specialised, but not very highly developed, eyes, or to have been 



