224 THE STUDY OF ANIMAL LIFE CHAP. 



the shells are dissolved before they reach such depths. 

 Here the floor is covered with a very fine reddish "or 

 brownish deposit, often called " red-clay," a very hetero- 

 geneous mixture of meteoric and volcanic dust and of 

 residues of surface-animals. Along with this, in some of 

 the very deepest parts, e.g. of the Central Pacific, there 

 are accumulations of Radiolarian shells, which do not 

 readily dissolve. 1 



Protozoa have many inter-relations with other forms of 

 life. Thus many devour other minute organisms, as well 

 as organic debris. Others again, such as Radiolarians, live 

 in partnership (symbiosis) with unicellular Algfe. Many 

 form the fundamental food of small animals of higher 

 degree, such as Copepod Crustaceans. They form an 

 important part of the " stock " of the sea-soup on which 

 many surface animals depend. Moreover, among them 

 there are many parasites both on vegetable and animal 

 hosts. Protozoology has come to be a very important 

 sub-science, comparable to Bacteriology, for some of the 

 deadliest diseases are due to Protozoa. Thus certain 

 disorders of the human alimentary canal and liver are 

 due to species of Amoeba, the organism (Plasmodium or 

 Laverania) that causes malaria is a Sporozoon, and the 

 terrible tropical scourge known as sleeping sickness is 

 caused by an Int\isoTia,n(Trypanosoma) which is transferred 

 from the blood of some wild animal into man by the bite 

 of the tse-tse fly. 



2. Sponges. The first animals to be successful in form- 

 ing a body were the sponges. They have no organs in 

 the strict sense, but they show the beginnings of muscular, 

 connective, and some other tissues, i.e. combinations of 

 similar cells performing similar functions. 



Adult sponges are sedentary, and plant-like in their 

 growth. With the exception of the freshwater Spongil- 

 lidae they live in the sea fixed to the rocks, to seaweeds 

 and to animals, or to the muddy bottom at slight or at 

 great depths. They feed on microscopic organisms and 

 particles, borne in with currents of water which continu- 



1 For details, see conveniently H. H. Mill's Realm of Nature 

 (Lond.), revised edition. 



