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 I. 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 Entomostraca. 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 as a 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,, antennae, swimmerets, etc. 

 To the first segment of the head the movable and stalked 

 eyes are attached (Fig. 81, e). The next segment bears 

 the small and large antenna or feelers ; then follow six 



