NATURAL HISTORY OF AMERICAN LOBSTER. 245 



ARTERIAL SUPPLY OF THE SWIMMERETS. 



The dorsal or superior abdominal artery passes backward just above the intestine 

 and gives off six pairs of segmental lateral vessels, which, besides supplying the intestine 

 itself, send arterial blood into the great muscles of the tail, the posterior lobes of the 

 gastric glands, and the sexual organs. To complete the statement, however, it must 

 be added that the main branches of the lateral segmental vessels are curiously continued 

 around the sides of the body to the swimmerets or pleopods, which they feed with 

 arterial blood." 



The swimmerets have been invariably described as receiving their blood from the 

 inferior abdominal artery, both in the lobster and cra)dish, an error which may have 

 arisen in the first instance from failure to inject the vessels or from inference, proba- 

 bility favoring the inferior vessel, on the principle that organs as a rule draw their blood 

 supply from the nearest source. The error, started in some such way, has escaped the 

 scrutiny of such keen observers as Professors Huxley, T. J. Parker, and Howes, and is to 

 be found in all the text-books and Uterature deaUng with these forms. It can be seen, 

 however, without recourse to much dissection, that the inferior abdominal artery is 

 too diminutive and passes altogether too small a quantity of blood to supply the 

 swimmerets, which are the most active of all the appendages, excepting only the 

 respiratory plate or "bailer" of the second maxilla. 



The superior abdominal artery divides at the hinder border of the fifth somite 

 into two branches, which embrace the intestine where it gives off a short caecum on its 

 upper side, and which run backward and diverge to supply the sixth somite and tail fan. 



The principal artery of the big claw (pi. xl) traverses the lower side of the Umb 

 and gives off numerous branches to the muscles of the segments. In the fifth podomere 

 it sends off a shoot which enters the big claw, passes to the abductor muscle along the 

 inner border of the big tendon, and ends in the fine meat of the dactyl. The main 

 artery, upon entering the claw, again divides, giving rise to four branches, three of 

 which supply the big adductor muscle and the fine meat of the propodus, while the other 

 passes to the adductor muscle and divides, sending a branch to both dactyl and pro- 

 podus. The division to the dactyl is united by a cross branch to the vessel which 

 supplies the abductor and enters the propodus from the fifth joint. In the index and 

 dactyl the arteries ramify in tree fashion, and apparently break up into a lacunar 

 system of irregular spaces in the fine meat. From this situation the blood returns by 

 a large irregular channel and enters the sternal sinus, whence it reaches the gills. 



It has been shown by Emmel (97) that as the returning sinus of the great cheUped 

 passes the ischium or third podomere it is divided into two channels by a septum of 

 connective tissue. These dorsal and ventral sinuses, moreover, possess valves which 

 originate as folds from the septum and become operative to staunch the flow of blood 

 from the breaking joint the moment a claw is shot off (see p. 282). 



01 am indebted to Prof. Carl B. James for first directing my attention to this fact, which must have been noticed by other 

 teachers in the laboratory. 



