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, ete. ; 
histologv. the character of the tissues of the body; and embry- 
ology, t of the development of the animal from the 
embryo y. 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. 
