CLASSIFICATION 239 



phylum, the Porifera, or sponges that seems to lack in its devel- 

 opment and in its adult state anything that can be properly com- 

 pared with the digestive tract as found in all higher many-celled 

 animals. A similar condition exists in the small group of animals 

 that is here designated as the Phylum Mesozoa. Hence the 

 Metazoa may be divided into two groups, the Parazoa, which 

 includes the Porifera and Mesozoa; and the Enterozoa, which 

 includes all other Metazoa. 



Again, if the same question be asked for the Enterozoa, it appears 

 that the most fundamental difference between these forms is the 

 presence or absence of another ca\aty, the ccelome, which surrounds 

 the digestive tract. In some cases, as in the Trochelminthes and 

 Molluscoida, the coelome seems to have degenerated; while in 

 others, the Platyhelminthes and Nemathelminthcs, it may 

 never have existed. In the more important phyla, the Annulata, 

 Arthropoda, MoUusca, Echinodermata, and Chordata, there are 

 either unmistakable signs of its former existence or it is well devel- 

 oped, as in the famihar vertebrates. For this reason, the Enter- 

 ozoa may be divided, as the table shows, into the EnteroccEla, 

 including the Coelenterata, and perhaps the Platyhelminthes, 

 which have no coelome; and the Ccelomoccela, which possess 

 such a body cavity. The Nemathelminthcs, the Trochelminthes, 

 and the Molluscoida may be left as uncertainties; although they 

 are presumably forms in which the coelome has become modified 

 beyond clear recognition. In this manner the phyla may be 

 grouped in larger subdi\dsions according to the classifications 

 maintained by many zoologists. A tabulation showing how the 

 phyla of anunals are divided into classes appears on p. 241. 



The Basis and Meaning of Classification. — For practical pur- 

 poses of listing and arrangement, the classification by phyla alone 

 is sufficient; but classification has a significance in modern zoology 

 apart from its convenience as a cataloging system. The basis 

 of the classification here indicated is structure, and structural 

 resemblances are beheved to indicate evolutionary relationships. 

 ^Vhen one says that certain animals are all chordates, or coelen- 

 terates, or annulates, one means that they are to be regarded as 

 more akin to one another than to any other group. Hence, the 

 " natural " classification, which zoologists are now attempting 

 to ascertain upon the basis of structural resemblance, is in 

 reality a familij tree of animal life. If certain phyla are placed 



