CRUSTACEA 339 



with subequal rami borne distally upon a protopodite are compara- 

 tively rare and usually occur in highly specialized crustaceans (Cope- 

 poda, Cirripedia, Malacostraca), (2) that the biramous limbs of the 

 Nauplius and Lepidocaris are not primitive but adaptive, the relations 

 of the rami of the limbs of trilobites are problematical, and the ad- 

 mittedly primitive Branchiopoda possess phyllopodia, (3) that the 

 unjointed, turgid, lobed phyllopodium more nearly resembles the 

 parapodia of certain annelids in which the neuropodium is axial, than \ 

 the stenopodium resembles the normal biramous parapodium. 



Concerning the functions of particular members of the series of 

 limbs, and the corresponding modifications of their structure, little 

 can be said that would hold good throughout the subphylum. There 

 is an immense variety in these respects. The antennules and antennae 

 are primarily sensory, and perhaps usually possess something of that 

 function when they are also capable of swimming, prehension, attach- 

 ment, etc. In the Nauplius larva (Figs. 235, 258) the antennules are 

 uniramous and the antennae biramous, and they normally retain these 

 conditions in the adult. The mandibles always play, by means of their 

 strong gnathobase, some part in preparing the food, whether by 

 chewing or by piercing for suction, but the distal part of the limb 

 {palp) may aid in locomotion or set up feeding currents. They gener- 

 ally lose in the adult the biramous condition which they have in the 

 Nauplius. The maxillules and maxillae tend to be phyllopodia. The 

 maxillules have usually the function of passing food to the mouth but 

 may serve other ends. The maxillae have various functions in con- 

 nection with feeding and respiration. The limbs of the thorax perform 

 in various cases practically every function for which appendages are 

 used. If a crustacean walks, it is usually by means of these limbs. 

 Often in one or more of them the last joint can be opposed to the joint ' 

 which precedes it, forming a chela (or a subchela), so that the ap- 

 pendage is adapted for grasping. Modification of the hinder thoracic 

 or anterior abdominal limbs in connection with reproduction is 

 common. Abdominal limbs are lacking save in certain of the Branchio- 

 poda and most of the Malacostraca. When they are present they are 

 commonly used for swimming, for setting up currents of water, or 

 for carrying eggs and young. 



Three elements of minor importance complete the external make- 

 up of the Crustacea. In front of the mouth is a labrum or upper lip ; 

 behind the mandibles is a lower lip or metastoma, usually cleft into 

 a pair of lobes known as paragnatha ; and on the telson usually (but 

 in no adult malacostracan except the Leptostraca, Fig. 267) is a pair 

 of caudal rami forming the caudal /w;t«. 



Appendages which are lost are regenerated at subsequent moults ; 

 and the highest members of the group possess an elaborate mechan- 



