8 STRUCTURE OF THE VERTEBRATES 



different, though both are used for flying. But the wings of the 

 bat and the bird are not only analogous but skeletally homol- 

 ogous. They are functionally similar and evolved from the 

 same ancestral vertebrate limb. 



Classification of Chordates. Like all other phyla, chordates 

 are classified according to the Linnean sj^stem. The heterogeneous 

 assemblage included in a phylum must be divided into smaller 

 and more coherent groups according to their fvmdamental struc- 

 tures. The simplest method of classification is to split the phylum 

 into the necessary number of Classes; each class into Orders, 

 an order being a fairly large group of animals with numerous 

 similarities of development and structure; the order is in turn 

 divided into Families; a family into Genera, a genus being a 

 coherent small group, and the genus includes a varying number 

 of Species. The species is the smallest group that can be accu- 

 rately set apart from others. 



It is often impossible to carry out a scheme as simple as the 

 above and maintain accuracy. Particularly is this true in a 

 diverse group like the chordates. It is necessary to set up Sub- 

 phyla or Sub-classes which may include only a few forms. An 

 illustration is a Sub-phylum of chordates which contains less 

 than a dozen species. These few species are so closely related 

 that they form only one family; but as a group they differ so 

 widely in structure from all other chordates that they must be 

 given an independent position equivalent to that of the verte- 

 brates, which contains six classes and several hundred orders. 

 In technical works it is often necessary to include numerous 

 other divisions in an attempt to keep equivalent groups in a 

 proper balance. As these divisions often become confusing even 

 to the technically trained they have been omitted as far as pos- 

 sible, for they are of more academic than practical interest. 



The following are the major divisions of the phylum with the 

 classes of each. Under the separate chapter headings a more 

 detailed classification will be given, but in each case the author 

 has taken the liberty of excluding any order not considered 

 necessary for the continuity of the discussion. 



