CHAPTER III 
CRUSTACEANS AND RELATED FORMS 
41. Crustaceans. — The Crustaceans (kriis-t a/shuns : 
Latin, crusta, crust) are so called because of their hard 
outer covering. They belong in the same group of ani¬ 
mals as the insects. The body consists of a limited number 
of segments, each of which usually 
bears a pair of jointed appendages. 
The appendages are variously modi¬ 
fied ; some aid in swimming, others in 
securing food, and others are used in 
walking. The jointed appendage is the 
characteristic expressed in the techni¬ 
cal name Arthropoda (ar-throp'o-da: 
Greek, arthros, joint; pod, root of pous, 
foot) given to the group to which all 
these animals belong. 
42. Crayfish. — As a typical crusta¬ 
cean we have the common crayfish, or 
Figure 48. — Crayfish, “ cra b” as it is known away from the 
Showing Eggs. , n . . 
seashore. The crayfish has nineteen 
pairs of appendages adapted to different kinds of work. 
It lives in fresh-water ponds and streams where there is 
sufficient lime for its use in building up its outside covering 
(exoskeleton). 
The animal is divided into two regions, the head-thorax 
region and the abdomen. The segments of the abdomen 
are clearly defined, but those of the head-thorax are so 
fused that they cannot be made out. 
43. Appendages. — The appendages of the head-thorax 
region are the most important to the animal. Certain of 
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