244 BULLETIN OF THE BUREAU OF FISHERIES. 



and muscle fibers," and has an arched roof and floor, with sloping sides. This chamber 

 lies close to the back, so that if the shell is perforated anywhere in the cardiac area 

 the animal will quickly bleed to death. The convex floor of the sinus covers the 

 sexual organs and the digestive gland, while at the sides only the thin shell of the 

 body wall (inner epimeral surface) separates the sinus from the upper part of the 

 branchial cavity. Moreover, the extensor muscles of the tail virtually pass through 

 the sinus and are inclosed between its sides and floor. 



The heart beats rythmically and heat accelerates its action. Plateau (214) found 

 that the isolated lobster's heart, when placed in a moist chamber, would beat for nearly 

 an hour; according to this investigator the movements of the decapod heart are governed 

 as follows: (i) By a cardiac nerve which arises in the stomato-gastric ganglion and 

 ends in the heart muscle; (2) by gangUon cells within the tissue of the heart itself, by 

 means of which its automatic movements are maintained, and (3) by depressor nerve 

 fibers which moderate the heart's action, but the real courses of which are not known. 

 The brain is found to have no direct influence upon the action of the heart. 



THE ARTERIES. 



Of the five anterior arteries .the ophthalmic or cephalic runs along the middle 

 Une just beneath the shell, and makes straight for the brain, which it supplies, 

 together with the eye stalks (upper side) , giving off a few twigs to the stomach sac in its 

 course. The paired antennal arteries issue from the side of the ophthalmic, and in passing 

 forward along the surface of the gastric glands they give off numerous small branches 

 to the following organs: The glands themselves, the gastric muscles and walls of the 

 stomach, the sexual organs, the thoracic muscles, and the body wall, or the integument 

 of the carapace and the inner epimeral wall of the branchial cavity; finally the same 

 vessel sends twigs into the eyestalk, the antennule, the adductor mandibuU muscles, the 

 antenna, and the green gland which lies at its base. The paired hepatic arteries supply 

 the gastric glands. Both ophthalmic and antennary arteries are subject to considerable 

 variation in both the lobster and crayfish. (See fig. i, pi. xi,iv.) 



Two arteries issue from the hinder end of the heart, where it swells into a bulb, 

 namely the sternal artery, which passes straight down and penetrates the nerve cord, and 

 the superior abdominal artery, which suppUes the greater part of the tail. The sternal 

 gives off twigs to the sexual ducts before it swerves to pass the intestine, and entering 

 the ring formed by the long commissures between the fourth and fifth ganglia of the 

 ventral chain (somites xii and xiii), gains the ventral side, where it divides or gives 

 off a posterior branch, the inferior abdominal artery, which supplies a small part of the 

 ventral surface of the abdomen, but none of the appendages. The main branch of the 

 sternal, the inferior thoracic artery, runs forward under the nervous system, and sup- 

 plies the slender legs, the great forceps, and the mouth parts. 



a According to Dogiel iyz), the pericardium also contains blood vessels, which can be injected from the superior abdomiaal 

 artery, as well as nerves supplied by a trunk (nerve of Dogiel) which is given off from the ganglion of somite xn. The valves of 

 the heart are further regarded as properly sphincters, rather than of the bilabial or semilunar form. On the other hand the sternal 

 artery, of which the superior abdominal may be considered a branch, is provided with true valves of the bilabial type. 



