334 PERCY SLADEN TRUST EXPEDITION 



longitudinally into two parts* — an inner, which bears the flagellum and its stalk 

 (together forming the endocerite), and an outer, which bears the exocerite in the form of 

 the scale (scaphocerite). On this outer division stands also a forwardly-directed spine, 

 which may, as in Harpiliojosis, be very long and strong, or may be represented merely 

 by a slight projection of the joint. As this spine is not homologous with the stylo- 

 cerite of the antennule, standing indeed on a different joint, the same name should 

 not be applied to it. The scale has a broad, fringed expansion and a stout, un- 

 fringed outer border, which ends in a distal spine. Its most remarkable variation is 

 found in Typton, where it is reduced to a vestige, which does not reach as far as the 

 beginning of the flagellum, though it shows traces both of the expanded region and 

 of the distal spine. In the endocerite, the flagellum stands on a two-jointed stalk. It is 

 doubtful whether there is any justification for calling the joints of this stalk " ischiocerite " 

 and " merocerite." The flagellum, like those of the antennule, is shorter in the sedentary 

 than in the active forms. It is shortest in Conchodytes tridacnce and C. meleagrincB. 



The mouth parts show, on the whole, an absence of modification, either within 

 the subfamily or in comparison with those of related groups, which is rather striking 

 in view of the specialized nature of the food of many of the commensal forms — such, 

 for instance, as those which live with crinoids or lamellibranchs — consisting as this 

 must largely of minute organisms collected by the feeding currents of the host. The most 

 remarkable features exhibited by the organs in question are (1) the tendency which 

 appears in various genera to a broadening of some or all the joints of the third maxilliped, 

 (2) in the Pontdnia group, that the inner lacinia of the maxillule is very wide and hairy. 

 The connection of these features with the functions of the organs is discussed below. 



The mcmdible (Plate 52, fig. 1 and Plate 57, figs. 26 d — d'") is deeply cleft into two 

 diverging processes, both directed obliquely towards the median plane of the body. One of 

 these — the incisor process — is thin and ribbon-like, trending at its base downwards, but 

 curving inwards and at the same time twisting its outer edge forward, so that, while at 

 its base it is nearly vertical, with its width transverse to the body, at its free end it is 



* [June, 1917. These parts, of which the outer stands always a little forward of the inner, and in 

 some Decapoda, as Vpogehia, becomes almost wholly distal to it, probably represent the two components 

 (protobasipodite and metabasipodite) of the basipodite (symbasipodite). I have recently discussed the 

 composition of this segment (Proc. Zool. Soc. Loudon, 1917, p. 53). The homologies of the antenna 

 present a very difficult problem. ,It would appear from the case of the Mysidacea that the segment upon 

 which the green gland opens is the second of the primitive series, and in that case the first, or precoxa, 

 must in other cases have disappeared, by excalation or by fusion with the head or with the coxopodite. 

 This suggestion is supported by the fact that the maxillai'y gland of Stomatopoda also opens upon the 

 second segment of the limb to which it is adjacent (Proc. Zool. Soc. loc. cit.), and that the gonoducts of 

 the Decapoda, probably homologous with the ducts of the excretory glands, open upon what development 

 shows to be the second segment of the legs (except where, as in the crabs, the openings have secondarily 

 shifted to the sterna). If Hansen be right in interpreting as segments the inconspicuous structures which 

 undoubtedly exist in the antenna of Nebalia proximal to the first apparent segment and between those 

 which appear to be the second and third, then there is in that genus a very complete and instructive 

 retention of the primitive segmentation of the limb. The segments will be, in succession, precoxa (pleuro- 

 cerite), coxocerite, protobasicerite, metabasicerite, and the so-called ischiocerite and merocerite incompletely 

 fused. If, on the other hand, Hansen's view be not accepted, then the absence of exopodite and of visible 

 opening of the green gland leaves the homologies of the segments obscure, though the flexion of the limb 

 suggests that the incompletely double third joint belongs to the endocerite proper.] 



