PELAGIC FAUtfA. 121 



growth is not sufficient to maintain the animal life which appears 

 to be nourished by it, and he suggests that the deficiency may be 

 made good through a peculiar symbiotic relation which appears to 

 exist between certain lowly-organised plants and animals. Thus, 

 many of the radiolarians and comb-bearers (Ctenophora) contain, 

 embedded in their body-substance, a number of yellow starch-cells, 

 which Brandt recognises as unicellular algse (Zooxanthellse), and 

 which are supposed to thrive upon the waste products of the ani- 

 mal, and to yield to it in turn the compounds elaborated in the 

 process of its own development. The relation of mutual benefit 

 which is here stated to exist probably requires further investigation 

 before it can be accepted as an absolute fact.* 



One of the distinctive characters of the majority of pelagic ani- 

 mals is their transparency, which renders them very nearly invisible 

 on the surface of the water. The nerves, muscles, skin, and organs 

 generally, are alike hyaline, although in many instances the liver 

 has remained unaffected. The protection thus afforded to such 

 animals as the radiolarians, jelly-fishes, tunicates, and many crus- 

 taceans, is compensated for in animals less transparent by a colour- 

 ing which harmonises with that of the open sea. Thus, the pre- 

 dominating colour is either blue or violet, as we find it in the 

 Portuguese man-of-war, the Velella and Porpita, and in lanthina 

 and Glaucus among the snails. The fishes are principally steel-blue 

 above and lustrous white underneath, a want of correspondence 

 which is also manifest in other animal groups. Glaucus, just men- 

 tioned, whose progression is effected in the manner of the common 

 pond-snail, ventral surface uppermost, has this side coloured blue, 

 and the opposite, or dorsal side, silver-white. Exceptional cases 

 are presented of an extreme brilliancy of colour, as in the copepod 

 Sapphirhina, which is said to rival in metallic lustre the humming- 

 birds, and to display the colours of the spectrum with the intensity 

 of the gleam of the diamond. 48 



Most of the pelagic animals, except the lowest, are devoid of a 

 shell, or, when present, the shell is usually very thin and fragile, 



* Professor Hensen estimates that in some parts of the Baltic there are 

 upwards of 140,000,000 plants (Rhizosolenia, Chsetoccros) in every ten cubic 

 metres of water, and maintains that this prodigious quantity is produced in 

 the course of about two months. " Bulletin of the United States Fish Com- 

 mission," August, 1885. 



