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. 



93 



