ARTHROPODA. 93 



part of the stomach. To this is jointed a pre-pyloric sclerite (pr), 

 forming the stem of the J_, which takes a downward and back- 

 ward course in the back wall of the constriction above-mentioned, 

 and joins the end of the uro-cardiac sclerite. At the junction 

 of the two there is a red-coloured median c n 



tooth (*), which projects into the cardiac 

 chamber. The diagram here shown indicates 

 the relation of these five sclerites as seen 

 from the side. 



From the extremity of the cardiac sclerite on either side a 

 small ptero-cardiac sclerite (pt) runs back, and unites with a zy go- 

 cardiac sclerite (z) that runs forwards from the end of the pyloric 

 scleriie. The inner side of each zygo-cardiac sclerite is thickened 

 into a red elongated lateral tooth (f), which projects into the 

 cardiac cavity, and is marked by numerous transverse ridges. 



Two anterior gastric muscles (28, a.g.m) take their origin on the 

 inner side of the carapace, and run backwards and downwards to 

 be inserted into the cardiac sclerite, while two posterior gastric 

 muscles (p.g.m) run downwards and forwards from a similar origin 

 to be inserted into the pyloric sclerite. 



By the contraction of the anterior and posterior gastric muscles 

 the cardiac and pyloric sclerites are pulled away from each other, 

 which involves (1) the conversion of the hexagon into a rect- 

 angle, causing the lateral teeth to approach; (2) the pulling out 

 of the sharp fold made by the urocardiac and prepyloric sclerites, 

 so that the median tooth passes downwards and forwards. Hence 

 the three teeth meet together in the centre, and effectually chew 

 anything that comes between them. The elasticity of the frame- 

 work then comes into play and separates the teeth. 



In addition to the sclerites forming the gastric mill, others of 

 less importance are found in the cardiac wall ; and in summer a 

 round button-like calcareous gastrolith is often seen projecting into 

 the cardiac cavity on either side. 



The communication between the cardiac and pyloric cavities 

 is very narrow, partly owing to the external constriction and 

 partly to hair-fringed projections. This is the commencement of 

 the strainer, the rest of which is formed by hairy cushions project- 

 ing into the pyloric cavity and reducing it to a fissure, anchor- 

 shaped in cross-section. A valve composed of five flaps separates 

 the stomach from the mid-gut. 



