302 THE INVERTEBRATA 



swimmerets of Astacus (Fig. 211 G) and the trunk limbs of Apus 

 (Fig. 213), there are many Umbs which depart more or less from 

 either type in the direction of the other — as, for instance, from 

 the stenopodial type in the shape of the exopodite (Fig. 243 B), or, 

 as stated above, in the relation of the latter to the rest of the 

 limb, or from the phyllopodium in the proportions of the rami or 

 the reduction of the endites. 



The comparison just made between the phyllopodium and the 

 stenopodium leaves untouched the question which of them is the 

 more primitive, that is, more resembles the limbs of the ancestral 

 crustacean. On this point there is an old and as yet unsettled con- 

 troversy. As proof of the primitive ness of the stenopodium it is 

 pointed out (i) that this limb is more widespread than the phyllo- 

 podium, (2) that it occurs in the Nauplius larva (p. 314), the 

 early phyllopod Lepidocaris (p. 322), and the trilobites, in all of 

 which it is likely to be primitive, (3) that it more nearly approaches 

 the form of the majority of parapodia of the Annelida, from 

 which the Crustacea are held to have taken origin. In demonstration 

 of the ancestral nature of the phyllopodium it is urged (i) that 

 typical stenopodia with subequal rami borne distally upon a proto- 

 podite are comparatively rare and usually occur in highly specialized 

 crustaceans (Copepoda, Cirripedia, Malacostraca), (2) that the bira- 

 mous 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 admittedly 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. 224, 248) 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 

 generally 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 



