THE ARTHROPODA 



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A to D. Diagram of model gastric mill which can easily be made. After W. 

 E. Roth, A, Cardboard as first cut out ; B, Model complete at rest ; C, Model 

 complete ; muscles contracted ; D, Median vertical section of model to show folds. 



Instructions : 



Cut out a piece of card shaped as in Fig. A. Along ab, cd, ef, hi, and tnn 

 cut just the surface of the card with a penknife ; do the same, but on the opposite 

 face of the card, along gk and lo. Then bend slightly downwards the triangular 

 pieces 2, 2 ; turn 9, 9 under the piece 6, 5, 6 until the lower surfaces of 9, 9 are flat 

 against that of 6, 5, 6: stitch the shaded part of 9, 9 firmly by thread or fine 

 wire to 6, 5, 6 ; then bend the unshaded part of 9, 9 till at right angles to the 

 shaded part, using lo as hingeline. These projecting pieces of 9, 9 then represent 

 the lateral teeth. 



Next bend the piece 1, 8, 4 upon hinge-line gk, until the shaded portion is 

 flat upon the surface of 4, where it must be securely stitched ; this done bend back 

 1, 3 on hinge-line cf until 3 is at right angles to 4. The projecting end of 4 made 

 prominent by these folds represents the central tooth. The piece 1 must now 

 be bent gently downwards upon 3, using cd as hinge-line, and 4 must be bent 

 sharply on 5, using mn as hinge-line. Lastly, perforate the corner of 6, 6 and 

 of 2, 2, and by a single wire (to allow a certain amount of rotation) unite right 

 hand 2 to right hand 6, and left hand 2 to left hand 6, in each case 2 being 

 outside 6. To do this 6, 5, 6 must be bent like a bow, its right and left arm* 

 being thrust downwards and inwards. The model will then be as in Fig. B. 



If now the pieces 8, 8 and 7, 7, which represent the anterior and posterior 

 gastric muscles, are pulled so as to represent the effect of a muscular contraction 

 the three teeth come sharply together, but are separated again and the whole 

 model brought back to its original condition by the elasticity of the cardboard. 

 Of course in the actual stomach of the crayfish the gaps between the ossicles are 

 filled in with thin, flexible chitin. By carefully adjusting the size and direction 

 of the 3 teeth in the model and further by hardening them with sealing-wax or 

 similar material, they may be made to grind bread, etc., into small fragments. 

 A sectional view is shown in Fig. D. 



E. Stomach or "gastric mill" of the crayfish cut through the middle, e. 

 cardiac regions of stomach ; d.L, duct from the liver ; g, gastrolith, or calcareous 

 disk secreted by the walls of the stomach ; i, intestine ; l.t., lateral teeth of grind- 

 ing apparatus ; m.t., median tooth ; oe, oesophagus ; py, pyloric region ; v, valve be- 

 tween cardiac and pyloric regions of stomach. (After Hatschek and Cori.) 



