NATURAL, history OF AMERICAN EOBSTER. 
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 crayfish, 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 literature dealing 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 limb 
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 cheliped 
passes the ischium or third podornere 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). 
oI 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. 
