THE CRAYFISH 251 



and cuticle turn inwards at the mouth and line the gullet 

 and proventriculus, which are together known as the fore- 

 gut. The mid-gut is lined with soft endoderm, and the 

 hind-gut is again lined with epidermis and cuticle. Thus 

 the regions often called stomach and intestine in the cray- 

 fish do not correspond with those so named in the frog 

 and earthworm, being lined with ectoderm, not endoderm. 

 The cuticle in the gut is for the most part thin, but in 

 places in the proventriculus it forms stout plates or 

 ossicles, certain of which bear strong teeth which project 

 into the forepart of the organ. By the action of muscles 

 these can be brought together to crush the food. The 

 whole apparatus is known as the gastric mill. 



Two large plates lie across the roof in the two divisions, and are 

 known as the cardiac and pyloric ossicles. They are joined in the 

 middle by two smaller pieces, the urocardiac and prepyloric ossicles, 

 which lie respectively in the front and hinder walls of the pit between 

 the two divisions. From the lower end of the prepyloric ossicle there 

 projects into the proventriculus the forked middle tooth. When the 

 mill is at rest the pit passes backwards, so that the prepyloric ossicle 

 in its hinder wall is also directed backwards under the pyloric, and 

 its tooth points backwards. At each side of the pit the cardiac and 

 pyloric ossicles are connected by two more pieces, the zygocardiac 

 ossicle, which articulates behind with the side of the pyloric, and the 

 pterocardiac ossicle, which joins the front end of the zygocardiac to the 

 side of the cardiac ossicle. These side ossicles do not run straight, but 

 slope outwards to meet at an angle, so that the outline of the whole 

 framework of the mill is roughly hexagonal. Internally each zygo- 

 cardiac ossicle bears a large, ribbed, lateral tooth. Anterior and 

 posterior gastric muscles run from the cardiac and pyloric ossicles 

 respectively to the carapace. When they contract they pull these 

 ossicles apart. The result is that ( I ) the upper end of the prepyloric 

 ossicle, being pulled backward by the pyloric, stands upright, thus 

 turning the middle tooth forwards ; (2) the zygocardiac and pterocardiac 

 ossicles are straightened out, so that the lateral teeth are brought 

 together in the middle line. Thus all three teeth meet inside the pro- 

 ventriculus. The ossicles are brought back to their former position 

 partly by the elasticity of the walls of the proventriculus and partly by 

 the contraction of cardiopyloric muscles (Fig. 160). 



The filter-chamber is also complicated, having various 

 internal ridges covered with bristles which serve to strain 

 out the particles of the food, so that only the finely crushed 

 matter passes into the mid-gut. Into this opens on each 

 side the liver or hepatopancreas, a large, lobed, yellow 



17 



