30 ESSENTIALS OF BIOLOGY 



9. ON UNICELLULAR AND MULTICELLULAR 



ORGANISMS 



The primary grouping of all organisms, whether plant or 

 animal ones, is into such as have a body constituted by a single 

 organic cell (see Section go) and such as have a body constituted 

 by a number (usually a very large number) of organic cells. The 

 unicellular organism, in general, is called aprotist ; or a. protozomi, 

 if it is regarded as an animal, and a protophyte, if it is regarded as 

 a plant. 



There is not a rigid distinction between unicellular and multi- 

 cellular organisms. Usually the above definition — one or many 

 cells in the body — holds good, but sometimes the body of a protist 

 is constituted by a small number of cells. Where this is the case 

 the cells are to be regarded as forming a colony of similar protists, 

 for they are not diiferentiated except in that some of them may 

 reproduce while the others do not but simply exercise ordinary 

 organic functions. 



Protists are always relatively minute in size, varying in diameter 

 from a few thousandth-parts of a millimetre to about one or a 

 few millimetres. That is, there is a limit of size to which a protist 

 may grow — why an unitary piece of organized matter can only 

 attain a very small magnitude is a curious and unsolved problem. 



The diversity in outer form and habits, and of internal bodily 

 structure is very great. Thus a man, a fish, an insect, a cuttlefish 

 and a sponge are remarkably different animals in every respect. 

 There are many hundreds of thousands of kiftds of plants and 

 animals and the structural differences between these particular 

 kinds of organisms include a bewildering mass of detail the 

 description of which requires a large library, containing the 

 transactions of most of the learned societies of the world. But 

 most of this detail is unessential for the purpose of this book 

 (though all of it may be necessary in the construction of a rational 

 classification of organisms). From our point of view it is sufficient 

 to survey the animal kingdom broadly. We can readily make a 

 relatively small number of principal types of animals, each type 

 being characterized by its general bodily structure and mode of 

 life. In the series of types considered below the rational classifica- 

 tion of animals — that arrangement which seeks to present a 

 summary of blood-relationships, lines of descent, or evolutionary 

 history, will not be strictly followed. It will present, rather, a 



