SUB-KINGDOM IX. THE PROTOZOA. 



ANIMALCULE. 

 BY HENRY M. BERNARD, M.A., F.L.S., ETC., AND MATILDA BERNARD. 



THE Protozoa, or first animals, are distinguished from all others by the fact 

 that they are unicellular organisms, i.e., the body of a Protozoan consists of 

 a single cell, whereas the bodies of all other animals, or Metazoa, are built up 

 of many cells. The one-celled Protozoan is capable of all the vital functions 

 of feeding, moving, breathing, and reproducing itself. In the Metazoa, the 

 greater number of the living cells of which the body is composed are 

 stationary in relation to one another, and the vital activities of the whole 

 have to be considered apart from the lives of the individual cells building it 

 up. We accordingly find these cells arranged into organs and tissues, each 

 specialised for some function necessary to the life of the whole. 



The Protozoan, then, is a free-living cell essentially like the cells which, 

 organised into great masses, build up the bodies of all other animals. 



All the Protozoa are of microscopic size, and are therefore removed from 

 ordinary observation. This is the more to be regretted since they compare 

 favourably with other classes of animals in variety and beauty of form, and 

 probably surpass all others in actual number of existing individuals. At 

 least five thousand species of these minute creatures are known, swarming, 

 at the present day, in salt and fresh water or in other animal organisms. 

 There are, in addition, several thousand fossil forms, the latter representing, 

 of course, only those Protozoa of past ages which were provided with a shell 

 or skeleton capable of preservation. Countless millions of such shells form 

 the chief constituent of vast areas of limestone rocks all over the world's 

 surface, and, in some more or less transformed condition, enter into the com- 

 position of a large part of the earth's crust. Vast numbers, again, are daily 

 being deposited on the sea-bottom to form the rocks of future ages. Nine- 

 tenths of the ooze over the greater part of the North Atlantic consists of 

 the shells of Protozoans, and one ounce of sand from the Naples shore yields 

 no fewer than one and a half million of their shells or parts of such shells. 



The very lowest of the Protozoa, that is, the very simplest of all animals, 



called the Monera, are mere microscopic particles of living jelly or protoplasm, 



in which, so far, no definite structure has been discovered. 



The Monera. In all other Protozoa, however, some differentiation of 

 the protoplasmic body has been found. Among many 

 minute granular particles scattered about in the jelly, there is always one 

 body (sometimes more than one) somewhat larger than the granules, and 

 different in character from the rest. This usually rounded body is known as 

 the nucleus. 



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