CHAPTER XII 



THE SUBPHYLUM CRUSTACEA 



Arthropoda, for the most part of aquatic habit and mode of respira- 

 tion ; whose second and third somites bear antennae ; and their fourth 

 somite a pair of mandibles. 



The Crustacea are essentially aquatic arthropods. That fact alone 

 makes it possible that in them the same appendages should combine 

 the functions of locomotion (by swimming), feeding (by gathering 

 particles from the water), respiration (by exposing a thinly covered 

 surface to the medium), and the reception of sensory stimuli. There 

 is perhaps no extant crustacean in which all four functions are thus 

 combined — unless we may regard the trunk limbs of the Branchiopoda 

 (see below) as sense organs in a minor degree — but not uncommonly 

 three, and perhaps usually two, are performed by the same limb. In 

 the lowest members of the subphylum — the "phyllopod" Branchio- 

 poda (such creatures as the fairy shrimp, Chirocephalus, shown in 

 Fig. 236) — a long series of somites of the trunk bear similar append- 

 ages which all function alike in swimming, respiration, and the gather- 

 ing of food. Evolution within the crustacean group appears to have 

 proceeded mainly by the specialization, for particular functions, of 

 particular appendages of an ancestor which possessed along the 

 whole length of the body a numerous series of limbs, of which all, 

 except probably the first pair (antennules), were as much alike and 

 capable of at least as many functions as those which the Branchio- 

 poda now possess upon the trunk. Such a condition existed in the 

 Trilobita, but in all modern Crustacea the appendages of the head 

 are already specialized for various uses, and in most members of 

 the group the specialization has gone farther. Moreover, it has taken 

 place in more than one way. Limbs which in one crustacean are adapted 

 to some particular function are in others specialized for quite different 

 services. 



Two other factors, added to, or perhaps consequent upon, the 

 specialization of limbs, have taken part in bringing about the great 

 variety of organization which exists in the Crustacea. One is a short- 

 ening of the body. As the efficiency of the limbs increases by speciali- 

 zation, there occurs a lessening of their number, and finally the re- 

 duction or loss of the somites whose limbs have thus disappeared. 

 The reduction, which has occurred independently in every class, has 

 taken place in the hinder part of the body, though as a rule the ex- 

 treme hind end (telson) is relatively unaffected. The other factor is 



