MOUXn-PAKTS OF TIIK PAL/EMONID PRAWNS. lyl 



flanged to the next, the liilges ax-e more regular, the valleys 

 between them bear spines, and at the hinder end there is not, fis 

 in Calanus, a single long, fringed spine, but a pair. Probably 

 the biting-surfaces of tlie mandibles of Galanus and Cypris are 

 houjologous with one another and with that of Lejndurus. The 

 mandible of Lepas (text-figs. 26, 36) is a veiy different structure, 

 thin, with a sharp, toothed edge, and in side view very similar 

 to the incisor processes of certain Decapoda. It is crossed near 

 the base by an articulation, and it seems possible that its toothed 

 edge represents, not the gnathobase, but the second endite. On 

 the other hand, the niaxillule of the barnacle (text-figs. 22, 29), 

 which appeal's to be built on the same plan, is of a fair width at 

 the end, and crossed by short ridges, each bearing a row of five 

 spines, so that this limb forms a sort of transition from the 

 ridged condition of Ci/prls to t\\n,toi Lepas. Unless the mandible 

 of the barnacles be homologous with the incisor process, the latter 



Text-figure 32. 



End view of mandible of Desmocaris. 

 For lettering see p. 71. 



is a structxu'e peculiar to the Malacostraca, and not to be found 

 even in rudiment upon the mandible of any other crustacean. 

 Two modes of origin can be suggested for it. It may be regarded 

 either as an outgrowth from the gnathobase or as representing" 

 the second endite. On the whole, the Ia,tteris the more plausible 

 view. From its first appearance in Rehalia (text-figs. 27, 31) 

 this process has a knife-edge, quite tinlike the ridged surface of 

 the molar process and its homologues, although it often comes to 

 bear a row of teeth. As has been shown, there is no trace in 

 p>hylogeny of its origin from the proximal part of the limb, and, 

 though in ontogeny (as, for instance, in that of Penceus or the 

 Stomatopoda) it and the molar process may originate from the 

 same simple precursor, this is no more than the differentiation, 

 in the course of development, of the rudiment of the appendage, 

 and happens also in the case of the palp. If this view be correct, 



