Unicellular and Multicellular Animals 



To begin at the beginning of our tale, we may ask ourselves 

 what are the lowest, simplest, living things that are known. 

 The question does not admit of any very definite answer. For 

 as we look around among a number of the most simple forms, 

 we find ourselves handicapped in our attempt to judge between 

 them, by a lack of knowledge of their nature. We come upon 

 organisms so small that they appear, even under the most powerful 

 microscope, only as the tiniest specks ; whose size is to be measured 

 in hundredths of thousandths of an inch. We even find good 

 evidence that living things exist which we are unable, in any 

 manner whatsoever, to see. Among the smallest known forms, 

 and also among some of the larger, we find organisms that we can 

 only describe as practically structureless, that appear as specks 

 of almost homogeneous protoplasm ; but it seems reasonable to 

 suppose that this appearance is due rather to our imperfect 

 observation than to an actual absence of differentiation. 



It is certain, however, that the lowest of the great groups is 

 that of the one-celled organisms. As all the higher types are 

 built up of large numbers of cells, essentially similar to those 

 which constitute the unicellular forms, it is important that we 

 should know something of the nature of this organic unit. A 

 typical cell is illustrated in Fig. 16. It consists of a mass of 

 protoplasm, with a distinctly differentiated portion called the 

 nucleus. The function of the nucleus is that of directing and 

 controlling the activities of the cell ; if it is removed, the remaining 

 portion of the cell soon dies ; while, on the other hand, a small 

 portion of the cell, if it contains the nucleus, may frequently 

 live, and build up new protoplasm to replace what was lost. Cells 

 are formed only from previously existing cells, by a process of 

 division, which is usually simply one of halving. This process 

 is begun in the nucleus ; it undergoes a complex rearrangement 

 of its parts, the object of which appears to be to insure an absolute 

 equality in the halves, and finally divides in two. The bulk of 

 the protoplasm then separates into two portions, a portion remain- 

 ing round each of the nuclei. The process of cell division is 



illustrated in Fig. 17. 



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