MOUTH-PARTS OF THE PALJ2MOXJD PRAWNS. 



57 



flanged to the next, the ridges are more regular, the valleys 

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

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

 the biting- surfaces of the mandibles of Calanus and Cypris are 

 homologous with one another and -with that of Lepidnrus. The 

 mandible of Lepas (text-figs. 26, 36) is a very 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 gnathoba.se, but the second endite. On 

 the other hand, the maxillule 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 Cypris to that of Lepas. Unless the mandible 

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



Text-figure 32. 



End view of mandible of Desmocarts. 



For lettering see p. 71. 



is a, structure 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 latter is the more plausible 

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

 this process has a knife-edge, quite unlike 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 

 phylogeny 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, 



