CHAPTER IX 
ARTHROPODS. CLASS CRUSTACEA 
92. General characters.—In the Arthropods, that is, the 
crabs, lobsters, shrimps, insects, spiders, and a vast host 
of related forms, the body is bilaterally symmetrical, and 
is composed of a number of segments arranged in a series, 
as in the earthworm and other annelids. A hornlike cu- 
ticle, sometimes called the shell, bounds the external sur- 
face—in early life thin and delicate, but later relatively 
thick, and often further strengthened by lime salts. Along 
the line between the segments this coat of mail remains 
thin and forms a flexible joint. Appendages also are borne 
on each segment, not comparatively short and fleshy out- 
growths like the lateral appendages of many of the worms, 
but usually long and jointed (hence the name Arthropod, 
meaning jointed foot), and variously modified for many 
different uses. 
93. Classification The species belonging to this group 
outnumber the remainder of the animal kingdom. Their 
haunts also are most diverse. Some are adapted for lives 
in the sea and fresh water, others for widely different sit- 
uations on land, and a great number are constructed for a 
life on the wing. A certain resemblance exists among them 
all, but the modifications which fit them for their different 
habitats are also profound, and have resulted in the division 
of the Arthropods into five classes. The first class (Crus- 
tacea) contains the crayfish, crabs, etc.; the second (Ony- 
chophora) includes the curious worm-like peripatus (Fig. 
98 
