PERIVISCERAL CAVITY AND ALIMENTARY SYSTEM 205 



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. Into the 

 mid-gut opens on each side a large, lobed, yellow gland, often 

 called liver or hepatopancreas, consisting of numerous short 

 tubes joined by ducts which finally communicate with the mid- 

 gut by an opening on each side. It is, however, much more than 

 a gland, as the finer particles from the fore-gut are directed into 

 it by the filter chamber and are digested in it ; it is best known 

 by the collective name digestive diverticula. The roof of the mid- 

 gut is prolonged into a short blind gut or caecum. Food is either 

 raked up by the third maxillipeds or seized by the chelipeds and 

 torn up by them and the smaller pincers. It is passed forwards 

 by the jaws to the mouth, where pieces are cut from it by the 

 mandibles and thrust by the mandibular palps and the maxillules 

 into the mouth. It is chewed in the proventriculus, and partially 

 digested by a protease sent forward from the diverticula, before 

 being passed into the diverticula themselves for further digestion 

 and absorption. The cuticle of the gut is shed with that of the 

 body. Shortly before a moult two fiat calcareous bodies, known 

 as * crabs' eyes ' or gastroliths, are laid down in the forepart of 

 the proventriculus. They are ground up before the moult takes 

 place. It is uncertain whether they consist of matter removed 

 from the armour of the body to weaken it in preparation for the 

 moult or are a store of material for the strengthening of the new 

 cuticle. Possibly they serve both purposes. 



BLOOD VESSELS 



The heart (Fig. 139) is a hollow organ with thick, muscular 

 walls. It is roughly hexagonal in outline, as seen from above, 

 and lies in the thorax, above the hind-gut and immediately 

 below the cardiac region of the carapace, in a space, known as 

 the pericardial sinus, with membranous walls, to which the heart 

 is connected by six fibrous bands called the alae cordis. Three 

 pairs of valved openings or ostia admit blood from the pericardial 

 sinus to the heart : one pair is dorsal, another lateral, and the 

 third ventral. From the front end of the heart arise three vessels 

 — a median ophthalmic artery, which runs straight forwards 

 over the proventriculus to supply the eyes and other organs of 



