THE GASTRIC MILL. 57 



connected by a bent jointed middle bar. As all these 

 parts are merely modifications of tlie 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 con- 

 traction and relaxation of these two pair of muscles, the 

 4 



