LIFE, ITS PHYSICiU. BASIS iVND SIMPLKST KXPIU-SSK )\ .'^3 



of which shall not bo recognized as siifTiciontly specialized (o he 

 called either plants or animals, but simply organisins. J^ut 

 this suggestion seems to meet with little practical favdr frnr,. 

 students of systematic biology. 



For a basis, therefore, of any study of the evolufinn f.f lif.., 

 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 hfe together with an account 

 of certain less simple or slightly complex or- 

 ganisms (Figs. 22-26) wliose physiology and 

 structure reveal successive stages in organic 

 complexity and specialization, and as tlie spac(> 

 in this book is Hmited, 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 sliglitly com- 

 plex organisms. 



The differentiation and growing com- 

 plexity of the body of those many-celled 

 animals wliich differ from and are, we mav 

 say, beyond and higher than the simple many- 

 celled forms, are by no means always along 

 the same Une (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 Echincdermata; 

 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. 

 of these phyla there is a fundamental or type structure (Fi_ 

 27). All of the Echinodermata, for example, arc buih on the 

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

 arms radiating from a central disk. Tlie Arthro]-)0(ls are all 

 animals with a bod}^ 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 



^jiir 



Tie. 



23. — Paranift- 

 ciitm aureliti. At 

 each OIK I there is 

 a contractile vac- 

 uole, ami in the 

 con tor is one of 

 (he nuclei. (After 

 \'erworn.) 



Xow for each 



