94 ANIMAL FORMS 
66); the third (Myriapoda, meaning myriad-footed) em-. 
braces the centipeds and “thousand-legs”; the fourth (Jn- 
secta) contains the insects; and the fifth (Arachnida) in- 
cludes the scorpions, spiders, and mites. 
94. The Crustacea.—The number of species of crusta- 
ceans is estimated to be about ten thousand, and while the 
greater number of these are marine, many are found in 
fresh water and a few occur on land. In size they range 
from almost microscopic forms to the giant crabs and 
lobsters. They differ also in shape to a remarkable degree, 
but at the same time there is a decided resemblance through- 
out the group, except in those species which have become 
modified by a parasitic habit. The characteristic external 
skeleton is invariably present, and gives evidence of the 
deep internal segmentation of the body. In the simple 
Crustacea this is very apparent, but in the higher forms it 
is usually more or less obscured, owing to the fusion of some 
of the different segments, especially those of the head, as in 
the crayfish (Fig. 59). 
The class of the Crustacea is subdivided into two sub- 
classes (Hntomostraca and Malacostraca), the first containing 
the fairy-shrimps (Branchipus, Fig. 53) and their allies, the 
copepods (such as Fig. 54), the barnacles (Fig. 55), and a 
number of other species. In their organization all are com- 
paratively simple, usually small, and the appendages show 
relatively little specialization, The other subclass contains 
the more highly developed and usually large-sized Crustacea, 
among which are the shrimps, crayfishes, lobsters, crabs, 
and a number of other forms. 
95. Some simple Crustacea—While the members of the 
first subclass are minute and inconspicuous, several species 
are often remarkably abundant in our small fresh-water 
pools. Among these is the beautifully colored fairy-shrimp 
(Branchipus, Fig. 53), with greatly elongated body and 
leaf-like appendages, whose relatively simple character leads 
the zoologist to think that they are among the simplest 
