Eyes of MoUuscs and Arthropods. 717 



A simple form of lieliophag Ì8 a red pigment spot usually situ- 

 ated near the animai pole of a single cell (zoospores and Protozoa). In 

 such cases, no nerve fibre8('?)are present, and therefore these receptacles 

 of energy are situated near, or in the centre, of animai activity, that is 

 in the centre of the greatest consumption of energy. In the Coelen- 

 terates they are necessarily situated near the nervous system, for the 

 latter is simply an aecumulation of ganglionic cells arising from the 

 ocelli and other sense organs to be spoken of later. The ocelli consist 

 of colorless and pigmented cells. The latter are, in nearly all cases, 

 pigmented at their outer ends, and if we suppose, as we have good 

 reason to , that the same method of nerve endings prevails here as in 

 the Mollusca, then there will be the re ti a t er minali a of nerve fibres 

 over the outer ends of the pigmented cells, and therefore exactly in the 

 Position necessary to receive the light Stimuli given to the pigment, and 

 .carry them to the necessary centres. 



We often hear it said of any pigmented spot that it is not an eye, 

 but simply a meaningless collection of pigment. No good reason can 

 be given why this should be called a mere pigment spot, and that an 

 eye. We cannot say that one is a pigment spot on account of the ab- 

 sence of nerves, and the other is an eye spot because it is supplied with 

 nerves, because it is to be presumed that nerve fibres of some kind 

 are everywhere present in the skin; and moreover the iustances in 

 which actual nerve endings have been proved to be present or ab- 

 sent in pigmented areas of doubtful meaning are extremely few. For 

 instance , one of the pigmented cells in the skin of Beroe or of Phro- 

 nima would be considered as pigment spots, but an exactly similar spot 

 on the head of a worm or of a larvai Mollusc would be an ocellus ! I think 

 it would puzzle those who believe that pigment is a waste product to 

 aitempt to define the difference in the function of a pigment spot like 

 that found in the skin of Phronima, or in the velum of a larvai Poly- 

 gordius, and a simple eye spot or ocellus like that in some Protozoa, or 

 in larvai Worms, Molluscs, etc. 



In the development of ganglionic, from sense hair cells, we have 

 said that the former were sense cells too delicate for the direct recep- 

 tion of the coarser vibrations, and therefore they were modified for 

 conducting sensations, and other scuse cells, for the direct reception of 

 them. This process is somewhat modified for the reception of the 

 most delicate of ali vibrations, that of light, in that in the higher 

 forms it is probable that the nerve fibres alone receive the light vi- 

 brations. 



