20 INTRODUCTION 



upon muscles or coriaceous or calcareous coverings. The verte- 

 brates are first represented in the fish-like forms. Bilateral sym- 

 metry, however, or the uniform arrangement of parts on each 

 side of a central axis, exists in several groups which are below 

 the vertebrates, the first pronounced example being found in 

 worms. Groups lower than worms have their organs arranged 

 around a central axis or radiating from it, and were once all 

 classed as radiates. 



An animal is classified in accordance with its morphology, 

 anatomy, histology, and embryology. Morphology determines 

 its general shape, the position of its limbs, eyes, and mouth, and 

 the covering of its body ; anatomy, the arrangement of its internal 

 organs, such as the position of its heart, lungs, stomach, etc. ; 

 histology, the character of the tissues of the body ; and embry- 

 ology, the method of the development of the animal from the 

 embryo to maturity. It is only after these exact discriminations 

 have been made that the groups are arranged. Owing to the 

 greater accuracy resulting from histology and embryology 

 (methods which have been employed only in later years), many 

 changes in classification have been made, and as science advances 

 will continue to be made. 



The primary groups are based on broad general characteristics, 

 but their divisions and subdivisions are determined by closer 

 distinctions. Animals having shells differ from those having a 

 cartilaginous or those having a crustaceous covering, and are 

 placed in different groups. Yet mollusks having a single or a 

 double shell, having spiral or flat forms, living on land, in fresh 

 water, or in the sea, while differing from one another, are all of 

 one group. Lobsters and crabs, although both have crustaceous 

 coverings, are very unlike ; and again, there are many species of 

 both lobsters and crabs. 



To group individuals, noting resemblances as well as differ- 

 ences, a system of classification has been arranged with the fol- 

 lowing divisions : 



Kingdom, Phylum, Class, Order, Family (or Suborder), Genus, 

 Species. 



