each side of them, there is one of these long lateral teeth. [Illustration: FIG. 10.—_Astacus fluviatilis._—Longitudinal section of the stomach (× 4), _c_, cardiac ossicle; _cæ_, cæcum; _c.p.v_, cardio-pyloric valve; _cs_, cushion-shaped surface; _hg_, hind-gut; _hp_, aperture of right bile duct; _lp_, lateral pouch; _lt_, lateral teeth; _mg_, mid-gut; _mt_, median tooth; _œs_, œsophagus; _p_, pyloric ossicle; _pc_, pterocardiac ossicles; _pp_, prepyloric ossicle; _uc_, urocardiac process; _v_^1, median pyloric valve; _v_^2, lateral pyloric valve; _x_, position of gastrolith; _zc_, zygocardiac ossicle.] There are two small pointed teeth, one under each of the lateral teeth, and each of these is supported by {56} a broad plate, hairy on its inner surface, which enters into the lateral wall of the cardiac chamber. There are various other smaller skeletal parts, but the most important are those which have been described; and these, from what has been said, will be seen to form a sort of hexagonal frame, with more or less flexible joints at the angles, and having the anterior and the posterior sides {57} connected by a bent jointed middle bar. As all these parts are merely modifications of the hard skeleton, the apparatus is devoid of any power of moving itself. It is set in motion, however, by the same substance as that which gives rise to all the other bodily movements of the crayfish, namely, _muscle_. The chief muscles which move it are four very strong bundles of fibres. Two of these are attached to the front crosspiece, and proceed thence, upwards and forwards, to be fixed to the inner face of the carapace in the front part of the head (figs. 5, 6, and 12, _ag_). The two others, which are fixed into the hinder crosspiece and hinder lateral pieces, pass upwards and backwards, to be attached to the inner face of the carapace in the back part of the head (_pg_). When these muscles shorten, or contract, they pull the front and back crosspieces further away from one another; consequently, the angle between the handles becomes more open and the tooth which is borne on their ends travels downwards and forwards. But, at the same time, the angle between the side bars becomes more open and the lateral tooth of each side moves inwards till it crosses in front of the middle tooth, and strikes against this and the opposite lateral tooth, which has undergone a corresponding change of place. The muscles being now relaxed, the elasticity of the joints suffices to bring the whole apparatus back to its first position, when a new contraction brings about a new clashing of the teeth. Thus, by the alternate contraction and relaxation of these two pair of muscles, the {58} three teeth are made to stir up and crush whatever is contained in the cardiac chamber. When the stomach is removed and the front part of the cardiac chamber is cut away, the front cross-piece may be seized with one pair of forceps and the hind cross-piece with another.