ORGANIC EVOLUTION 



231 



through a series of forms of the utmost diversity in appear- 

 ance, and how parts essentially alike may be adapted to the 

 most diverse ends. 



The Malacostracan body, be it an amphipod, an isopod, a 

 decapod, or what not — is composed of a series of twenty* 

 segments, each of which is essentially of the skeletal plan 

 shown in the diagram (fig. 142), except that appendages of 

 the foremost segment are typically unbranched and the 

 hindmost segment (the telson) is rudimentary and bears 

 no appendages at all. Some of these segments may become 



fused together and consolidated on 

 the dorsal side, only the appendages 

 and ventral margins remaining free. 

 This may occur at either end of the 

 body, but it occurs constantly in 

 the five front segments, these by 

 fusion forming the head. The ap- 

 pendages of these five segments 

 always consist of two pairs of 

 antennae at the front, one pair of 

 mandibles beside the mouth, and 

 two pairs of maxillae following the mandibles. These parts 

 and their functions will readily be understood because of 

 their likeness to the parts bearing the same names in the 

 insects already studied. Immediately following the maxillae 

 are one or more pairs of maxillipeds, likewise directed for- 

 ward beneath the mouth to assist in the manipulation of 

 the food. Then follow legs and swimmerets in more or less 

 variety, the terminal joints of some of the legs being modi- 

 fied in many cases into highly specialized grasping organs 

 called chelipeds, and the swimmerets being frequently 



Fig. 142. Diagram of a cross 

 section of the skeleton of 

 a typical body-segment of 

 a malacostracan, with its 

 appendages; b, basipo- 

 dite; e x, exopodite; en, 

 endopodite. 



*This is not counting a vestigial segment in the head region, 

 that is discoverable only during embryonic life, and with which 

 we have here no concern. 



