LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 33 



of which shall not be recognized as sufficiently specialized to be 

 called either plants or animals, but simply organisms. But 

 this suggestion seems to meet with little practical favor from 

 students of systematic biology. 



For a basis, therefore, of any study of the evolution of life, 

 an acquaintanceship with the life and struc- 

 ture of the simplest organisms is a necessity. 

 As the authors have already tried in another 

 book ("Animal Life") to present a simple 

 account of this life together with an account 

 of certain less simple or slightly complex or- 

 ganisms (Figs. 22-26) whose physiology and 

 structure reveal successive stages in organic 

 complexity and specialization, and as the space 

 in this book is limited, the authors must 

 refer their present readers to chapters I, II, 

 and III of " Animal Life ' for an account 

 of the life of the simplest and slightly com- 

 plex organisms. 



The differentiation and growing com- 

 plexity of the body of those many-celled 

 animals which differ from and are, we may 

 say, beyond and higher than the simple many- 

 celled forms, are by no means always along 

 the same line (Figs. 27-37). It is familiar 

 knowledge that animals can be classified or 

 grouped into a number of great divisions 

 called branches or phyla. For example, the 

 starfishes, sea urchins, sea cucumbers, etc., 

 constitute one phylum, the Echinodermata; 

 the crustaceans, insects, spiders, etc., con- 

 stitute another phylum, the Arthropoda, and 

 all the animals with a backbone or with a 

 notochord constitute another, the Chordata. Now for each 

 of these phyla there is a fundamental or type structure (Fig. 

 27). All of the Echinodermata, for example, are built on the 

 radiate plan. They recall the starfish with its five or more 

 arms radiating from a central disk. The Arthropods are all 

 animals with a body composed fundamentally of a series of 

 successive segments, some or all of these segments bearing 

 pairs of jointed appendages; and so on. We need not pursue 



FIG. 20. Paramce- 

 cium aurelia. At 

 each end there is 

 a contractile vac- 

 uole, and in the 

 center is one of 

 the nuclei. (After 

 Verworn.) 



