THE MEANING OF EVOLUTION 247 



divide all animals into about nine of these phyla — the 

 Protozoa or unicellular organisms ; the Porifera or 

 sponges ; the Coelenterates, a group which includes all 

 such organisms as Zoophytes, Corals, Sea-Anemones, 

 and " Jelly-fishes ; the Platyhelminth worms, that is 

 the Tapeworms, Trematodes, and some other struc- 

 turally similar animals which live freely in nature ; 

 the Annelids, a rather heterogeneous assemblage of 

 creatures which includes all those animals commonly 

 called worms ; the Echinoderms, which are the Star- 

 fishes, Sea-Urchins, and Feather- Stars found in the sea ; 

 the Molluscs, that is the animals of which the Oyster, 

 the Periwinkle, the Garden-Slug and the Octopus are 

 good examples ; the Arthropods, which include the 

 Crustacea, the Insects, and the Spiders ; and lastly 

 the Vertebrates. Any such classification we naturally 

 endeavour to make as complete a one as possible, but 

 round the bases of the larger groups there cling small 

 groups of organisms the precise relationships of which 

 are doubtful. Yet, on the whole, these sub-kingdoms 

 of organisms represent clearly the main directions along 

 which the present complexity of animal structure has 

 been evolved. 



There is an essential structure which we endeavour 

 to assign to all the animals of each phylum, and which 

 is different from the structure of the animals belonging 

 to all other phyla. The Protozoa, which for the present 

 we regard as animals, are organisms the bodies of which 

 consist of single cells. These cells may become aggre- 

 gated into colonies, but they may as well exist apart 

 from each other. They may be enclosed in limy, 

 siliceous, or cellulose skeletons or shells, or they may 

 possess limy or siliceous spicules in their tissues — these 

 parts are non-essential, and the schematic Protozoan 

 is a cell containing a single nucleus, and capable of 



