18 SEAWEEDS 



them, but there appears to be little doubt of their 

 plant nature. Mixed with these also in temperate 

 seas are the Coccospheres (Fig. 585), and inhabiting the 

 warmer seas of the tropics the EhabdoepheTes (Fig. 

 58a), organisms of highly probable plant nature, but 

 less studied even than the Peridiniccc. Their broken- 

 down parts known to geologists as Coccoliths and 

 Rhabdoliths are, like the remains of Diatomaccce, 

 known from the chalk, and now play an important 

 part in laying down the deep-sea deposits of non- 

 polar seas, associated in this (as also in life) with the 

 animal Foraminifera of the globigerina oozes. Min- 

 gled with these organisms there is a profusion of 

 pelagic Protophyta, which sometimes, as in the case of 

 Trichodesmium erythrceum, form great banks dis- 

 colouring the ocean over large areas, and in their 

 origin resembling the fresh-water phenomena known 

 as the " breaking of the meres " in Shropshire, and 

 described by de Candolle and others as occurring in 

 the Lake of Morat and other places, by which large 

 sheets of water are tinged green or reddish owing to 

 the colossal multiplication of minute fresh-water 

 Alga?. Such occurrences have been often noted in 

 the ocean, and, though ordinarily inconspicuous, the 

 Algaa that cause them, and other allied forms, are 

 always present in considerable numbers, as disclosed 

 by the use of the tow-net. Other organisms of 

 abundant occurrence in blue water are Pyrocystis 

 noctiluca (Fig. 57), a source of the brilliant luminosity 

 of tropical seas ; Halosphttra viridis (Fig. 55), of a 

 wider range in warm and temperate seas ; and other 

 Protococcacecv. The investigation of this pelagic flora 



