ARTHROPODA 
HIS subdivision of the animal kingdom includes insects, cen- 
tipedes, spiders, and crustaceans, which together constitute 
more than half the known species of animals. Although these 
animals are so unlike in general appearance, it is easy to recog- 
nize the common characteristics which place them together in 
one group. The name Arthropoda, meaning “jointed-footed,” 
suggests perhaps one of the most obvious points of resem- 
blance. The Arthropoda have bilateral symmetry, one side of 
the body being like the other; they are covered with a horn-like 
material (chitin); they are divided into segments; the segments 
have appendages; and the appendages are jointed so as to admit 
freedom of motion. Their manner of growth is peculiar; they 
cast off their rigid external coverings and secrete larger coats of 
mail, and at these periods increase in size or undergo meta- 
morphosis. 
There are such modifications of these general features as the 
habits of the different species demand. For instance, the appen- 
dages may be constructed for walking, swimming, boring, sucking, 
or the seizure and preparation of food. In some animals the 
appendages form a part of the breathing-organs, in others are 
used as organs of sense. Every detail of the organism, down to 
the hairs, has its special use and function. 
CLASS CRUSTACEA 
The crustaceans vary in size from microscopic minuteness to 
two feet or more in length. The giant crab of Japan (Kaempferia 
kaempferi) exceeds this, being commonly from eight to twelve feet 
244 
