Structure of the Mouth in Sucking Crustacea. 297 



those of both Eleutherognatha and Trochalognatha in this 

 essential particular, that they do not touch each other with 

 their apices, but leave a broad space between them, whilst in 

 front thej fit close against a transversal arched kind of bolster, 

 thicker towards both extremities, and belonging to the upper 

 lip, but separated bj a deep and broad furrow from the bilobate 

 leaf of the upper lip, which is visible outside on the head. 

 From the centre of this transversal cushion, backwards to the 

 hypostoma, a narrow serrate seam appears, which the observer 

 in the first instance, without special examination, is inclined 

 to interpret as being formed by the interior margins of the 

 middle lobe of the lower lip touching one another in the 

 median line ; if, however, the lateral lobes of the lower lip 

 are now cut aAvay, it becomes evident that the middle lobes 

 are entirely absent, and that the serrate seam in question is 

 formed by the inner margins of the large flat grinding-teeth 

 of the mandibles, which consequently occupy the whole space 

 between the transversal cushion of the upper lip, the lateral 

 lobes of the lower lip and the hypostoma, thus entii-ely closing 

 up the palate from below. The latter does not appear until the 

 mandibles are opened ; and it is then seen that behind the 

 transversal ridge of the upper lip there is on the palate a low 

 semicircular eminence, and along the median line, just above 

 the serrate inner margins of the grinding-teeth, a narrow 

 depression forming a sort of canal, leading to the opening of 

 the pharynx. Outside and behind the lateral lobes and man- 

 dibular springs of the lower lip, the long and very high 

 stipites of the mandibles are seen, whilst their narrow, sharp, 

 edgewise-set outer lobes fit into the transversal furrow above 

 mentioned, between the foremost bilobate leaf of the upper lip 

 and its transversal ridge or cushion. Examined from the 

 fore end, after the removal of the anterior bilobate part of the 

 upper lip, the lobes of the mandibles show the form of two 

 short saws with curved blades — the arched edge, which is 

 coated with enamel as hard as glass, being cut into a row of 

 sharp saw-teeth, increasing gradually in size towards the 

 lower corners of the inner lobes, where the last two teeth, 

 particularly the lower one, are considerably enlarged and 

 developed into a pair of exceedingly sharp, incurved, prehen- 

 sile hooks. The lobe of the left side glides above the one of 

 the right side. If the parts are turned round and examined 

 from the back, we perceive that the right mandible is en- 

 tirely without inner lobe, whilst the left mandible possesses 

 one placed behind the upper half of the outer lobe, and of the 

 same structure, excepting that the teeth of the saw are all of 

 equal size and that there are no prehensile hooks ; the outer 



