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 Branchio- 

 poda (see below) as sense organs in a minor degree — but not un- 

 commonly three, and perhaps usually two, are performed by the same 

 limb. In the lowest members of the subphylum — the *'phyllopod" 

 Branchiopoda (such creatures as the fairy shrimp, Chirocephalus ^ 

 shown in Fig. 225) — a long series of somites of the trunk bear 

 similar appendages which all function alike in swimming, respiration, 

 and the gathering of food. Evolution within the crustacean group 

 appears to have proceeded 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 

 Branchiopoda 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. Indeed, the greater 

 part of the vast variety displayed by the several elements of the organi- 

 zation of the Crustacea is due to such specialization and to the fact 

 that it has proceeded along many different lines. This evolution, 

 which has been much affected by the presence or absence of the 

 enveloping skin fold known as the ''shell" or carapace (see below), 

 has involved, as the efficiency of the limbs has increased, a lessening 

 of their number, and finally the reduction or loss of the somites 

 whose limbs have thus disappeared. The reduction, which has oc- 

 curred independently in every class, has taken place in the hinder part 

 of the body, though as a rule the extreme hind end (telson) is rela- 

 tively unaffected. The transformation of the external make-up of the 

 body is of course reflected in the internal organization, which shows 



