238 CLASSIFICATION AND ORGANIZATION OF ANIMALS 



Phylum Trochelminthes, the rotifers. 



Phylum Molluscoida, the polyzoa, brachiopoda, etc.^ 



Phylum Echinodermata, the starfishes, sea-urchins, sea-cucum- 

 bers, sea-hhes, etc. 



Phylum Annulata, the earthworms and other segmented worms. 



Phylum Arthropoda, the crayfish and other crustaceans, the 

 insects, spiders, centipedes, etc. 



Phylum Mollusca, the clams, snails, squids, etc. 



Phylum Chor'data, the tunicates, etc., and the vertebrates (c/. 



p. 36). 



Within the Hmits of these major groups, and a few lesser ones 

 that are commonly appended to particular phyla, are included all 

 the manifold forms of animals. The diversity of animal Kfe, 

 which is so bewildering upon superficial examination, is thus seen 

 to be unified by the existence of only a few principal types of 

 organization. 



In many textbooks of zoology no broader classification is 

 attempted than this division of the Animal Kingdom into phyla. 

 It is possible, however, to make further unifications, although 

 some zoologists doubt the certainty of such relationships. To 

 illustrate what may be done in this regard, let us proceed as follows: 



Taking the Animal Engdom as a whole and assuming a knowl- 

 edge of its varied types, suppose one asks what is the greatest 

 difference between the various kinds of animals. If all the animals 

 were to be divided into two groups, on what basis would they be 

 separated? As previously explained in introducing the Protozoa 

 (c/. p. 153), one may answer that animals may be first subdivided 

 into Protozoa and Metazoa, as shown by the accompanying tabula- 

 tion (Fig. 117). Unicellular animals may thus be placed over 

 against all others, and the Phylum Protozoa may be written in the 

 right-hand column. 



Proceeding further, one may ask a similar question for the 

 Metazoa. What is the greatest difference between the kinds of 

 many-celled forms? Some zoologists would not agree to the 

 answer here given, but a majority would probably say that the 

 Metazoa may be divided into two principal groups, according as 

 they have or have not a digestive cavity. There is one large 



2 The Phylum Molluscoida is questioned because it includes forms so diverse. 

 There is much justification for its division into at least two phyla, the Folyzoa 

 and Tcntaculata. 



