THE CRAYFISH 



253 



gland, consisting of numerous short tubes joined by ducts 

 which finally communicate with the mid-gut by an v opening 

 on each side. The roof of the mid-gut is prolonged' into 

 a short blind gut or cmcum. 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 it is cut up by the 

 mandibles into pieces small enough to be swallowed. 

 These are chewed in the proven triculus, strained, and in a 

 finely divided state passed into the mid-gut. The juice 

 secreted by the liver digests all classes of food-stuffs, and 

 digestion and absorption take place within the liver as well 

 as in the mid-gut. The cuticle of the gut is shed with that 

 of the body. Shortly before a moult two fiat calcareous 

 bodies, known as "crab's eyes" or gastrolitks, 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. 



The heart is a hollow organ with thick, muscular walls. 

 • > ^ * s rou S n ty hexagonal in outline, as seen from 

 * J **'" ' 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 alee 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 the head, and a pair of antennary arteries, which 

 start one on each side of the ophthalmic, run forwards and 

 outwards, and divide each into two branches, one gastric 

 and the other to the antennae and green gland. Behind 

 and below the antennaries arise a pair of hepatic arteries, 

 which supply the liver, and from the hinder angle of the 

 heart there is given off a vessel which at once divides into 

 a dorsal abdominal artery, which runs backwards above 



