THE GASTRIC MILL. 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 imner 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 
