CHAPTER VII. 
SEVENTH BRANCH OF ANIMALS. 
CRABS AND INSECTS (Arthropoda). 
General Characteristics—Animals having jointed feel- 
ers, jaws, and legs, arranged in pairs ; skin hard, and body 
made up of rings or segments. The Arthropods are di- 
vided into two classes : first, crustaceans, crabs, etc. ; sec- 
ond, insects. 
Class 1.—CRABS, etc. (Crustaceans), 
General Characteristics.—Arthropods that breathe by 
means of gills attached to the feet, or in some cases respir- 
ing through the body-walls, as in the Zxtomostraca. The 
body is covered with a hard skin, composed principally of 
carbonate and phosphate of lime. This forms an external 
skeleton, protecting the soft body parts within.. 
Skeleton. —Taking the fresh-water cray-fish as an ex- 
ample (Fig. 81), the body is seen to be divided into two 
general regions: the cephalo-thorax (head and thorax) 
and the abdomen, and asa rule made up of twenty dis- 
tinct rings or segments often difficult to define. Upon 
these the organs or appendages are arranged in pairs, be- 
ing modified for various purposes, as cutting and crushing 
claws, paddles, stalked eyes, antenns, swimmerets, etc. 
To the first segment of the head the movable and stalked 
eyes are attached (Fig. 81, ¢). The next segment bears 
the small and large antennae or feelers; then follow six 
