THE CLASSIFICATION OF ANIMALS 



to establish the basic structure of the present-day system. Linnaeus was by 

 training more of a botanist than a zoologist, although he zealously studied 

 animals as well as plants and developed a classification that included most of 

 the animals known to him. His work with animals was less successful than his 

 botanical work; nevertheless, the fact that he was able to formulate a scheme 

 which became widely accepted, and which served as the basis for the modern 

 system, justifies his enduring fame in the field of biological science called 

 taxonomy (Greek, "law of arrangement"). Linnaeus' most significant con- 

 tributions to classification were (1) his use of structure as the basis of com- 

 parison between forms, and (2) his establishment of binomial nomenclature, by 

 which each organism is given a generic and a specific name. 



The Modern System. It is a familiar fact that animals fall into restricted 

 groups called species (singular species, not specie), which may be approxi- 

 mately defined as groups whose members successfully interbreed and resemble 

 each other more than they resemble the members of other similar groups. 

 The individuals, which are the ultimate material of classification, can thus be 

 grouped into species. Different species with much in common form genera, 

 similar genera form families, similar families form orders, and so on. We may 

 begin with the individuals and species, as did early investigators; or, as the 

 system is now established, we may begin with a larger group and follow its 

 subdivisions until the species and individuals are reached. For example, the 

 existence of a large group of animals known as Vertebrata was recognized after 

 it had been discovered that a great array of animals possessed backbones com- 

 posed of vertebrae. Later, it was found that several types of animals with- 

 out vertebrae, and with relatively simple organization, possessed certain 

 features in common with the vertebrates, particularly a notochord such as 

 occurs in the early embryos of all vertebrates. The Vertebrata were then 

 placed, along with these simpler groups, within a single phylum, one of the 

 principal subdivisions of the Animal Kingdom. Despite the fact that this 

 phylum, the Chordata, includes a very wide range of animal types, all 

 chordates have certain basic features in common. These common, distin- 

 guishing characteristics include the presence, in the adult or at some stage of 

 development, of (1) a primitive axial skeleton, the notochord, (2) gill slits, 

 or traces of these structures, and (3) a dorsal, tubular central nervous system. 



The principal subdivisions of the phylum Chordata are listed in Figure 7.2. 

 One of these subdivisions is the subphylum Vertebrata, including, among 

 others, the class Amphibia. Within this class, the existing forms may be 

 classified as follows: 



Class Amphibia (Fig. 7.1) 



Order Caudata, salamanders, newts, etc. 

 Order Apoda, limbless, worm-like amphibians 

 Order Salientia, frogs and toads. 



After the orders, proceeding to smaller groups, come families, then genera 

 (singular genus), and finally species. Thus, the order Salientia contains, along 

 with some half dozen other families, the family Ranidae, which in turn includes 



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