718 William Patten 



In the simplest heliophags , the red pigments play the most prom- 

 inent role. 



With the aecession of more uerve fibres , an increase in their sen-* 

 sibility, and a more advantageous arrangement, they are enabled to 

 receive the light vibrations directly, withoiit the Intervention of pigment. 

 It then becomes diffìcult to distinguish between pigment destined for the 

 reception of energy, and that for secondary, protective purposes. 

 What might , perhaps , furnish a rather uncertain criterion as to the 

 nature of the pigment, would be its solubility. Chlorophyll, as is well 

 known, is extremely unstable, and soluble in many fluids, even in water, 

 but more especially in alcohol. The pigment of the lower animala is 

 very often soluble in alcohol , the red pigment, almost without excep- 

 tion. The red pigment in the eyes of starfish is entirely dissolved by 

 alcohol (Carrière). In Cardium this is also the case as well as in Pecten 

 (at least sections oi Pecte?i eyes, killed only with alcohol, were colorless). 

 Sections of the red pigmented eyes of Euphausia^ treated only with 

 alcohol, are also colorless. The red coloring matter in the rods of the 

 Vertebrates. is destroyed even by a short exposure to the sunlight. But 

 it is well known that the pigment in certaiu compound eyes is extra- 

 ordinarily diffìcult to dissolve. Such pigment is undoubtedly of a pro- 

 tective nature, as we bave various reasons for believing. The pigment 

 spots of Phronima are dissolved with sublimate, so is also the brownish 

 red pigment on the wart-like papillae in the skin (òi Pterotrachea ^ as 

 well as the black pigment of the eye. It can be no mere accident that 

 the eye-spots of zoospores and of almost ali Protozoa, Coelenterates, and 

 Echinoderms, should be red as well as the tapetum of the eyes of Pecten. 

 Alciopa, and Euphausia. Wherever the eyes appear to be entirely 

 black , they will probably be found to contain a mixture of red and 

 black pigment, producing brown, or the red pigment will be found in 

 the rods alone. It also appears that the red color, like the reflecting 

 surfaces, is best developed in the eyes of nocturnal, or deep sea ani- 

 mals. 



The appearance of retinophorae, and the arrangement around them 

 of pigment cells, is an extremely primitive and universal arrangement. 

 In the Coelenterates, the retinophorae seem, from their shape and general 

 appearance, to be true scuse cells. It seems to be a general law that ali 

 sense cells are colorless. 



The most economie distribution of a given number of pigmented, 

 and sense cells upon a given area, so that each pigment celi should 

 touch a nervous one, would be the arrangement of the pigmented cells 



