3io DISCOVERY REPORTS 



short anterior marginal setae extend across the labrum towards the middle line (Fig. 10) and the short 

 posterior marginal setae interlace with the ventral setae of the basale (Fig. 7). The length of the palp 

 and that of the basis are such that during the downward arc of movement of the former, first the 

 long posterior marginal setae, secondly the apical claws and lastly the long anterior marginal seta 

 are drawn down the anterior margin of the carapace. At the posterior extreme of this movement, 

 the terminal claws lie between the palps of the maxillulae and the endopodites of the maxillae. The 

 posterior marginal setae lie with their tips pressed against the surface of the labrum. Finally the long 

 anterior marginal seta lies just inside the ventral margin of the carapace, with its apex within range 

 of the claws of the palp of the maxillule. 



The outstanding structural features of the mandible are the presence of a gnathobase on the basis 

 as well as on the coxa and the bicondylar articulation of the coxa. Skogsberg (1920, pp. 556-7) has 

 pointed out that the mandible is used for holding food fast and for mastication. With reference to the 

 incisor edge of the basis, he stated that as G. O. Sars (1887) pointed out, 'it seems to have the same 

 function as the cutting part on the mandible of many other Crustacea, while the pars incisiva of the 

 coxa serves as a sort of tuber culum molar e, to break up the food more finely'. He stated that the latter 

 process also serves partly as a ' cutting organ '. The bicondylar suspension of the coxa, which seems to 

 be surprisingly uncommon in the Crustacea, is important in that it will enable an extremely powerful 

 crushing action to be carried out by the gnathobases, while the firm proximal support provided to the 

 palp will facilitate its independent movement. The disadvantage is that the arrangement will strictly 

 limit the size of food-material which can be passed between the coxal gnathobases. Borradaile (1922) 

 has described how in the shore crab (which also has a bicondylar suspension of the mandible) the food 

 is first broken up by other appendages, before passing between the mandibles. In Conchoecia, it is 

 difficult to see how, with the possible exception of the maxillules, appendages other than the mandibles 

 could function in this manner. Quite clearly the shearing action of the gnathobases of the mandibular 

 bases must have the function of cutting food into pieces which will pass between the gnathobases of 

 the coxae into the oral atrium. This, as will be shown, is confirmed by the type of food material found 

 in the stomachs of the animals. 



The structure of the coxal gnathobase is complex and so also must be its function. It is interesting 

 to note its division into incisor and molar processes in a manner similar to that of the mandibular 

 gnathobase of the Malacostraca. There can be no doubt that the function of the incisor edges of these 

 structures in Conchoecia will play an important role in breaking up food material passed between 

 them. It is more difficult to visualize the function of the molar surfaces, with their closely packed 

 spines. These probably would serve to grip and crush food-material, particularly if this were small, soft 

 particles. The row of four teeth on the posterior margin of the molar surface must serve to retain 

 food-material in the oral atrium, which same function in the case of smaller particles would be 

 served by the fringe of fine spines. The fine orally directed spines will clearly aid transport of food- 

 material into the mouth. 



In his earlier work, Muller considered that the halocyprids might to a limited extent be bottom- 

 living and use their mandibular palps for walking, as do the cypridinids. It is now clear, however, 

 that they are planktonic during the whole of their life-cycle. This leaves the mandible free for its 

 important role in feeding. The palp with its well-developed apical claws, extending beneath the oral 

 atrium in the flexed position, and its wide arc of antero-posterior movement, is well adapted to grasping 

 food and bringing it within the range of action of the mouth-parts. In dissected appendages it is 

 difficult to see what function the marginal setae of the palp could possess in the whole animal, but 

 when the palps are flexed it is evident that they could aid retention of food beneath the labrum and 

 oral atrium. 



