128 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



walk upon their tiptoes, so to speak, and that their skeletons are very similar. 

 In recognition of these facts, which demonstrate that they are related to each 

 other, they are united into a larger group, called an order, in this case, the order 

 Carnivora. If we consider other animals familiar to us we find that they too 

 fall into natural orders; thus the cattle, sheep, deer, horses, resemble each other 

 in that they walk on their toenails, which are broadened into hoofs, their legs 

 are much modified for running, and their teeth are broad and ridged for grinding 

 up vegetable food. They constitute another order, the Ungulata. Similarly 

 the rabbits, squirrels, mice, rats, beavers constitute the order Rodentia, dis- 

 tinguished by sharp, chisel-like front teeth. 



If we further consider these and other orders of familiar quadruped animals 

 we find that they have certain large characters in common, such as that they are 

 all clothed with fur, that their young are born alive and nourished with milk, 

 and that their skeletons are quite similar. These facts point to undoubted 

 relationships between them, so that they are united into a still larger taxonomic 

 group, the class, in this case the class Mammalia, or the mammals. 



The birds are another natural class, distinguished by their covering of feathers, 

 modification of the fore limbs into wings, and egg-laying habit. Frogs, toads, and 

 salamanders are another, with slimy smooth skins; snakes, lizards, alligators 

 and similar animals form another class, with dry scaly skins; fishes constitute 

 a class distinguished by the presence of gills and fins. 



All of these apparently diverse classes of animals have further certain common 

 characters, such as the presence of an internal cartilaginous or bony skeleton, 

 consisting of skull, vertebral column, limb girdles, etc., two pairs of appendages, 

 a ventral chambered heart, a dorsal nervous system, etc. They therefore 

 together constitute one of the great divisions of the animal kingdom, a phylum. 



The taxonomic divisions are therefore variety, species, genus, family, order, 

 class, phylum. There are usually other subdivisions, also, as subphylum, sub- 

 class, suborder, superfamily, subfamily, etc. Naturally, the details of classi- 

 fication are not yet agreed upon because we know as yet little about the natural 

 relationships of animals and because it is difficult to decide whether certain 

 characters are as important as others, as, for example, whether certain differ- 

 ences between two animals will place them in different genera only, or whether 

 they are great enough to separate them into two different families. For this 

 reason the student need not be surprised to find that the various textbooks do 

 not agree on the details of classification, although all recognize the same large 

 groups. 



There follows a key to the phyla and classes of the animal kingdom. Peculiar, 

 rare, and aberrant forms are not provided for in the key, but an attempt has 

 been made to include all animals commonly met with. With the aid of the key, 

 classify ten different animals provided by the assistant. In this key each state- 

 ment of characters bears a number followed in parentheses by one or more 



