174 MR. R. I. PococK ON THE [June 17, 



camai'ostome ; and the inner edges of their lowei- surface form 

 a siniilai- hinge-like union with each other in the middle line, 

 this hinge appearing externally fi-om beneath as a longitudinal 

 groove passing between the two segments. These two hinges 

 pei'mit only of a restricted range of movement of the coxte in a 

 vertical plane, whereby the camarostome is compi-essed between 

 them. 



Originally, no doubt, the two apposed preaxial suifaces of the 

 coxfe, where they met beneath the mouth and camarostome, 

 persisted as a double partition separating the cavity of the coxa 

 of the right side from the cavity of the coxa of the left side. 

 But in existing forms this partition has disappeared, so that the 

 cavities communicate freely with each other, the muscles of the two 

 being contiguous in the middle line (text-fig. 41, A, ms., p. 173). 



The camarostome (rostrum, labrum) is large, broad in its basal 

 half, narrowed and depressed at the apex, and wedged in between 

 the coxfe of the chelfe, as described above. Its dorsal wall consists 

 posteriorly of a chitinous plate, so-called clypeus (text-fig. 41, 

 A, B, c/., p. 17?)), which is laterally hinged on each side, as already 

 stated, to the adjacent edge of the coxa, and is continuous posteriorly 

 with the membrane that foi-ms the anterior boundary of the 

 prosoma. Tliis membrane is folded foi-wards over the proximal 

 portion of this plate and closely applied to it ; and from the middle 

 of its area arises a stout, hooked entosclerite, wliich projects 

 backwards into the cavity of the prosoma (text-fig. 41, A, B, ent., 

 p. 173). Owingtothe overfoldingof this membrane and the closeness 

 of its contact with the horny plate {chjjieus), the latter appears 

 upon dissection to jut backwards into the body-cavity, and the 

 hook-shaj)ed entosclerite appears to be an upgrowth from the middle 

 of its dorsal suiface. Macei'ation in caustic potash, however, 

 reveals the ti-ue relations of the parts, and shows, further, that the 

 entosclerite itself is a hollow invagination of the integument and 

 unconnected with the horny plate. 



Beyond its point of union with the coxte, the camarostome is a 

 free, membranous, or weakly chitinized hairy lobe. Distally, it 

 is compressed and descends between the coxse, overhanging the 

 mouth and forming a flexible upper lip, hairy in the middle, and 

 encircled latei-ally and below with a fringe of close-set, perhaps 

 sensory hairs, which no doubt act also as a mechanical sieve, 

 as Bernard says, to strain the solid from the liquid elements of 

 the food. The cavity of the camarostome, which is irregularly 

 elliptical in transA'^erse section, is filled for the most part with 

 muscles which pass from its roof to its floor, the latter being the 

 dorsal wall of the entrance to the alimentary canal. When 

 the dorsal integument or roof of the camarostome is cut away and 

 the muscular tissue removed to display its floor, the latter is seen 

 to be formed like the bowl of a deep and pointed spoon, the short 

 handle of which is represented by the dorsal wall of the pharyngeal 

 portion of the foregut with which the floor of the camarostome is 

 posteriorly continuous. 



