GENERAL ZOOLOGY 



limb is similar in all. Structures in different animals which reveal, like the 

 fore limbs of vertebrates, basic similarities in organization and in embryonic 

 origin, are spoken of as homologous structures. In contrast, parts of animals 

 which are of diverse origin and composition, even though they may be 

 adapted to similar functions, are said to be anologous; the wings of birds 

 and the wings of insects are examples often cited. In classifying animals, 

 then, we depend not on the functional similarities of analogous parts but on 

 the more fundamental and significant structural similarities exhibited in 

 homologous features. 



At the present time, many species of animals remain to be cataloged, and 

 the taxonomic positions of known species are undergoing constant revision as 

 additional information and insight seem to warrant. Therefore the "family 

 tree" of living things cannot be drawn with certainty in all its features, 

 although many lesser conclusions can be drawn with reasonable accuracy, 

 considering the nature of the evidence. Upon this tentative basis the tree 

 of descent shown in Figure 7.3 is presented; but before considering in detail 

 what this figure means, we should examine the catalog of animal types now 

 known to us. 



The Principal Types of Animals. In accordance with the foregoing 

 principles of classification, by which animals are arranged in species, genera, 

 families, and so on, zoologists have arrived at the present comprehensive 

 system. Sweeping changes have been made, in the past, as knowledge of new 

 types or new facts about known types have accumulated. It appears, how- 

 ever, that classification of the major phyla and their main subdivisions now 

 rests upon a fairly permanent basis. 



During the earlier years of this century there was wide acceptance, at least 

 by American zoologists, of a system including 12 or 15 animal phyla, to which 

 lesser groups of questionable status and affinities were appended. More 

 recently, the tendency has been toward an increasing number of phyla, in 

 recognition of the distinctive features of many of the smaller groups. There 

 appears to be no serious objection to the elevation of a minor group to the 

 rank of a phylum if its characteristics are sufficiently different from those 

 of other animals to justify the change. Hyman, whose studies have done 

 much to elucidate possible interrelationships between invertebrate groups, 

 makes the following statement: "A phylum should consist of closely allied 

 animals distinguishable from any other phylum by well-defined positive char- 

 acteristics, some of which do not exist in any other phylum or not in that 

 particular combination. Any group of animals, however small, having such 

 distinct characters, should be regarded as a separate phylum until evidence 



'The term invertebrate is used generally to include animals without vertebrae, or backbones. 

 The distinction betv/een vertebrates and invertebrates, although convenient, is artificial from the 

 taxonomic standpoint. It cuts across the phylum Chordata, some members of v^^hich are vertebrates 

 and some invertebrates. It thus appears in contrast to the distinction between the phyla, or be- 

 tween the Protozoa and the Metazoa. 



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