Protozoa. 305 



certain distinct parts of the body devoted to a special 

 work; for instance, the hand is adapted to the work of 

 grasping ; it is an organ and its special work, or function, 

 is prehension. But the hand is composed of several kinds 

 of tissues. It is supported by bony tissue, the muscular 

 tissue gives motion to the fingers, the nerves are composed 

 of nervous tissue, connective tissue makes up the tendons 

 and part of the muscle, and the skin is another kind of 

 tissue. In addition there is usually some fat, which is 

 considered a still different tissue. The higher animals are 

 organisms, that is, they are made up of organs, each of 

 which has its special function to perform. In other words, 

 each organ works for every other organ in the organism, 

 and every other organ works for it. 



The Colonial Protozoa. This name is given to the 

 protozoans that have more than one cell, some of them 

 being actually many-celled. The simplest of these proto- 

 zoans are little more than an aggregation of cells, each of 

 which leads a nearly independent life, doing its own diges- 

 tion, etc. There is almost no division of labor among 

 them. Others form a true colony, and there is a rather 

 gradual series in development until in the higher forms 

 there is a division of labor approaching that found in the 

 simpler Metazoa. The cells in the simpler colonial proto- 

 zoans lead lives so nearly independent of their associates 

 that we might imagine the colony falling to pieces and 

 each cell again taking up an independent life, like that 

 of the amoeba. Not so, however, with the metazoans. 

 After their differentiation in structure and specialization 

 in function they are so modified that they could no longer 

 live independently. In developing one function each cell 

 has neglected other functions till now it is no longer able 

 to perform them. In other words, it has become dependent. 



