CH. Ill] LIFE IN THE SEA 75 



animals, and into the Protophyta, which are single-celled plants. 

 But this division is far from being an absolute one and there are 

 many forms which may be placed in either category. 



The Infusoria are Protozoa which play a more important 

 part in fresh-water than in marine plankton. Most of them are 

 littoral organisms living in the mud or on the surface of water 

 weeds. Only two groups of infusoria are of importance in the 

 sea. The Flagellates are represented by Noctiluca, a protozoan 

 which occurs at times in the sea to such an extent that the water 

 may be, to the naked eye, visibly discoloured by it. It is the 

 principal cause of the phosphorescence of the sea in some regions. 

 Its distribution is sometimes peculiar ; thus it occurs on the west 

 coast of England and Wales in enormous shoals, which have a 

 very wide distribution within the limits mentioned and occur 

 during the greater part of the summer and autumn. At times it 

 occurs in the inshore waters, even in the estuary of the Mersey 

 up beyond the Liverpool landing stage in such masses that the 

 sea ma}^ be brilliantly phosphorescent and a tow-net dragged 

 through it may contain this organism to the exclusion of ap- 

 parently everything else. 



The Peridinians, of which Geratium is the most common 

 genus, are shelled infusoria provided with flagella. Ceratiiun 

 is, like Noctiluca, also a very common cause of phosphorescence 

 of the sea. Allied to the Peridinians are the Ciliata, of which 

 the Tintinnoidae are the most common planktonic representatives. 

 They too are shelled animals, but in their manner of nutrition 

 are rather to be regarded as approaching to the unicellular plants. 

 Many of them are extremely elegant in form. Some are neritic, 

 or inshore forms, but many are oceanic in their habitat. 



The best known groups of the protozoa which occur in the 

 plankton are the Foraminifera and the Radiolaria. It is the 

 shells of these extraordinarily numerous animals which form the 

 wonderful deep-sea deposits, the Globigerina and Radiolarian 

 oozes. It was formerly thought that those organisms, the 

 shells of which made up the Globigerina ooze, belonged to the 

 benthos, but the discoveries of the Challenger proved beyond 

 doubt that the Foraminifera making up this deposit are animals 

 which when alive live in the open sea at a great variety of depths. 



