109 



THE MECHANISM OF THE GASTRIC MILL. 



According to Huxley* the movement of the gastric 

 mill is effected by means of both the anterior and posterior 

 gastric muscles. By the contraction of these muscles the 

 urocardiac tooth is thrown forward, and simultaneously 

 the zygocardiac teeth are rotated inwards and the three 

 teeth meet in the middle line. 



Mocquard has been fortunate enough to observe the 

 movements in a living Stenorhyncus having a remarkably 

 transparent carapace. He states that the active move- 

 ment is brought about almost solely by means of the 

 anterior gastric muscles. If the posterior muscles act at 

 all, it is only very feebly and spasmodically. When the 

 anterior gastric muscles contract, the urocardiac ossicle 

 and the median tooth are thrown forward. The movement 

 is slightly complicated because of the connection between 

 the posterior part of the urocardiac ossicle and the lower 

 part of the propyloric ossicle. When in a state of rest 

 the lower part of the latter ossicle lies considerably behind 

 its upper border. As a result of the contraction of the 

 anterior gastric muscle the lower part is drawn forward so 

 that the ossicle takes up a vertical position. The median 

 urocardiac tooth, if not in contact with the propyloric 

 ossicle, would have a simple backward and forward 

 movement. But the connection between the two ossicles 

 causes the median tooth to move in an arc the convexity 

 of which points downwards. 



Since the anterior gastric muscles are inserted on the 

 inner ends of the pterocardiac ossicles, the latter are also 

 drawn forward when the muscles contract. This move- 

 ment causes the outer ends of the ossicles to turn down- 

 wards and inwards. Because of the connection between 



* Huxley, T. H. The Crayfish. [International Science Series.} 



