Eyes of Molluscs and Arthropods. 711 



really do penetrate to the bottoni of the deep seas, causing the brilliant 

 coloration of the animals inhabiting those places. This suppositiou would 

 also account for the enormous dcvelopment of the eyes, or of the so- 

 called »luminous organs« of deep sea-fishes. 



Protozoa are highly eolored and contain pigment of all shades 

 from green and red to black. But this coloration cannot be regarded 

 as protective in the sense of making them less easily seen , since they 

 are so small as to be invisible, in most cases, to their enemies. 

 Their number does not depend hardly at all on their animai enemies, 

 but upon the amount of food and their power to withstand great 

 changes in the amount of beat and moisture. But we know that these 

 animals, as well as nearly all others, are greatly influenced by, or de- 

 pendeut upon the amount of sunlight; the effect it has upon the highest 

 Vertebrate animals is too well known to require any confirmation. We 

 cannot suppose that the light affects all parts alike, there mustbe some 

 Organs, or substances, particularly affected by it. Of all living organisms, 

 plants are most dependent upon the sunlight for their energy, and their 

 pigment, or Chlorophyll, is without doubt the substance affected by 

 sunlight. We know that animai pigment like that of plants is depen- 

 dent upon the sunlight for its existence, that it is extremely sensitive 

 to changes in the quality and quantity of light , and since animai pig- 

 ment cannot be regarded as a waste product, or as a protective cover- 

 ing , except in special cases, the only rational suppositiou is 

 that pigment is the substance in animals directly affected 

 by the sunlight. But what is this effect of the sunlight? It must be 

 either injurious, or beneficiai to the animai. The almost universal 

 dependency of animals upon the sunlight makes it absurd to suppose 

 this effect to be an injurious one. The experiments of Romanes 

 and Graber (81) show that many Coelenterates and Echinoderms are 

 heliotropic i. e. they select those places best provided with light. I have 

 myself noticed the same property in certain species of Ötarfish, Aste- 

 rias glacialis^ which always occupy the side of an aquarium next the 

 Window. A species of Ascidian, Clavellina, that I have often observed, 

 almost always has its free extremity directed towards the light. Pecten 

 Jacohaeics and P. opercularis, nearly always has its open side directed 

 towards the light ; I believe that the same is true of Cardium , Pec- 

 tunculus and Avicula, although my observations upon these three 

 genera were too scanty to be of much importance, It is well known that 

 very many Copepods and Cladocera show a decided preference for the 

 best illuminated side of a jar. The same is true of innumerable larvae 



