168 ORGANIC EVOLUTION CONSIDERED 



the one which is first differentiated. It is found in 

 the organisms belonging to the vegetable kingdom as 

 well as in those belonging to the animal kingdom."* 

 These so-called eyes of microscopic plants and of 

 animals are pigment spots. 



It is also claimed that around the rim of the jelly- 

 fish are " pigment spots," " eye-specks," or " ocelli," 

 consisting of pigment enclosed in little cavities. 



The pigment spots on the ends of the rays of star- 

 fishes are spoken of as rudimentary " eyes." 



It is said that each of the five " ocular plates " of 

 the Echinus bears an " ocellus," or rudimentary eye. 



The Pecten, a bivalve mollusk, has eyes that are 

 organs of vision, located in the edge of its mantle, 

 and they are similar to those of the snail in structure. 



The snail has two simple eyes located oh the tips of 

 its long tentacles. Each eye consists of " a globular 

 lens, with a transparent skiu (cornea) in front, and a 

 colored membrane (choroid) and a nervous net-work 

 (retina) behind." 



The Nauplii, larvae of Barnacles, and other forms, 

 have a median eye ; but after several moults they be- 

 come pupae with two compound eyes. Cyclops, one 

 of the Water-fleas, has a single large eye. 



Horse-shoe crabs have compound eyes and also 

 ocelli. The Eurypteridse, which became extinct in the 

 Paleozoic, had a pair of ocelli and two large marginal 

 compound eyes. 



The Isopoda generally have two eyes that are col- 

 lections of simple eyes, but sometimes they are 

 compound. 



Spiders have from two to eight simple eyes, which 



differ much in their methods of arrangement. The 



genus Nops has only two eyes, which are described as 



" large, black, glittering," and they are unique among 



* The Psychic Life of Organisms, by Binet, p. 23. 



