ARTHROPODA 



'47 



SUBTYPE I. CRUSTACEA 



The Crustacea (Eat. cntsta, shell) are essentially aquatic, 

 consisting of the crabs, lobster, prawns, shrimps, and similar 

 animals; they breathe by means of gills. A few, however, like 

 the land crabs, wood lice, and sand hoppers, live on the land. 

 Crustacea occur both in fresh and salt water, and range in size 

 from microscopic animals to huge crabs with legs over a meter 

 long. The shell or outer covering of the body is secreted by 

 the skin and consists of a chitinous substance, which is usually 

 impregnated with calcium carbonate, except at the joints or 

 boundaries between the segments. This hard shell has to be 

 cast off from time to 

 time in order that the 

 animal may increase 

 in size, and this pro- 

 cess, called ecdysis, 

 or moulting, takes 

 place at tolerably 

 frequent intervals 

 while the animal is 

 young, and less and 

 less frequently as the 

 animal grows older. 



The structure of 

 the appendages is of 

 considerable impor- 

 tance in the Crustacea. They are all built on the same type : 

 attached to the body is a basal piece, to the free end of which 

 two pieces or segments are attached, one on the side toward the 

 median line of the body, the other toward the outside, and these 

 two segments are known as endopodite and exopodite respec- 

 tively ; each may consist of several segments, end to end. Such 

 an appendage is called biramous (Fig. 139). The exopodite is 

 the swimming portion of the appendage, the endopodite is used 

 for walking, and so we find one or the other modified, rudimentary, 

 or altogether lost, according to the predominant method of loco- 

 motion in any particular species. The eyes of the Crustacea are 

 different from any that we have found up to this point. Typically 



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b 



c 



d 



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FlG. 139. Diagrammatic cross section through an abdom- 

 inal segment of a lobster, showing the hard parts. ,;. 

 tergum ; b, sternum; c, pleuron ; d, protopodite ; e, exo- 

 podite; f, endopodite. (Drawn by the author.) 



