-MOUTH-PAETS OF THE SHORE CRAB. 125 
apophysis, will pull that part of the limb upwards and so rotate the cutting- 
edge downwards and outwards. 
The sole movement of the head of the mandible is a rotation upon its 
articulation which alternately parts the incisor edges, opening them like a 
pair of doors, and brings them together. The palp is extended and flexed, 
digging its apex into the space between the incisor edge and the flank of the 
labrum. 
7. The Metastoma is a fleshy structure which lies behind the mouthy between 
the mandibles and the m:ixillules. It has two forvvardly-directed lobes, 
the paragiiatha, which stand wide apart against the mandibles, coverino- the 
notches on the hinder side of the latter between the head and the apophysis. 
The paragnatha are joined by a low transverse cushion, a little raised in the 
middle, on the border of the mouth. The whole is supported on each side 
by a sclerite which follows its base and at the end is sutured to the mandibles. 
The paragnatha contain some glandular tissue, but are not muscular and 
appear to be moved only passively. 
8. The Labrum is a lai'ge, fleshy lobe which forms the anterior border of 
the mouth. Its lase is rounded and swollen in front but narrows behind, 
where it projects into the mouth. Distally it is produced behind into a 
nose-like process which overhangs the mouth. Its exposed (ventral) wall is 
strengthened by a triangular sclerite. Its sides are moulded to fit closely 
against the dorsal faces of the mandibles when the palps are flexed against 
them. It contains, besides a good deal of glandular tissue, much muscle, 
notably in two longitudinal bands. Its movements are bard to observe, since 
it has not so sharp an outline as the limbs and is only exposed when the 
mandibles part, but they appear to consist chiefly in a tucking of the nose 
into the mouth and its withdrawal. 
9. The Mouth lies above the hinder end of the mandibles. It is a longi- 
tudinal slit, which forks in front owing to the projection into it of the base 
of the labrum, and behind owing to the low median prominence of the meta- 
stoma. To it converge all the surrounding structures — the nose of the 
labrum, the mandibular palps, the inflected hinder angle of the incisor edge 
of the mandible, the inner lacinia of the maxillule, and the pyramidal endite 
of the first maxilliped. All these are so formed as to make entry to the 
mouth easy but egress from it very difficult. 
III. 
1. The functions of the mouth-parts are threefold. They subserve respira- 
tion by keeping a stream of water flowing through the gill-chamber and 
hindering particles from lodging upon the gills ; they subserve alimentation 
by tearing up the food and thrusting it into the mouth ; and one of them, 
the third maxillipeds, cleans the eyes, antennules, and antennae. 
2. .Ihe branchial chamber of the crab is no less complex and specialized 
