INTEODUCTORY. 33 



ia no essential peculiarities from skore-haunting forms, can 

 live at a depth of more than five miles, is the fact that brilliant 

 coloration is so generally met with among them. 



This discovery obviously disposes of the old idea that colour 

 always depends for its development upon sunlight. Experiments 

 with sensitised photographic plates — a most delicate test — 

 show that the sun's rays can only penetrate for a few fathoms. 

 It is, however, believed by some naturalists that some rays of 

 light can reach even the greatest depths. Nevertheless there 

 seems to be one objection to this view, and that is the fact 

 that the sea-water contains too many opaque objects to permit 

 of any such passage of rays. 



The surface fauna consist of creatures which are, for the 

 most part, transparent or but faintly coloured, such as Medusae 

 of all kinds, minute Crustacea, many worms, such as Sagitta, 

 and various species of ascidians and molluscs, besides radio- 

 larians and other Protozoa, and a few plants. 



Some of these organisms are so transparent as to be invisible 

 when placed in a bowl of sea water ; but many of the more 

 highly developed, such as the Salpa, are to be easily detected 

 by the yellow " liver " or by the contents of the alimentary 

 canal. 



The surface waters often swarm with those minute Alga3 

 which have been termed Diatoms ; these are so abundant in 

 some oceans, and so far outnumber other pelagic organisms, 

 as to produce, by the constant raining down of their siliceous 

 skeletons, a characteristic deposit upon the sea bottom. 

 Diatoms are coloured yellow by a pigment belonging to the 

 chlorophyll series, which is somewhat opaque ; the Salpaj 

 and other surface animals feed upon Diatoms, and the yellow 

 colour of the alimentary canal is due to their presence in large 

 numbers. Now, the surface fauna are not confined to a thin 



