284 THE CRUSTACEA 



Posteriorly the heart sends off a median vessel, the superior 

 abdominal artery, while the unpaired descending artery (some- 

 times called the sternal artery) may arise separately from the 

 heart (Brachyura) or may branch off from the superior abdominal 

 artery just beyond the valves which mark its origin from the 

 heart. 



The descending artery passes on one side (either to right or left) 

 of the intestine and pierces the ventral nerve-chain in nearly all 

 Decapods, passing between the connectives uniting the sixth and 

 seventh thoracic ganglia. Only in some of the Brachyura, where 

 the concentration of the nervous system reaches its highest point 

 (Oxyrhyncha and some Brachyrhyncha), this perforation of the nerve- 

 mass does not take place, the artery passing behind instead of 

 through it. On arriving at the ventral surface the artery bifurcates 

 in the median plane, a large branch, to which the name of sternal 

 artery is commonly applied, running forwards to supply the ventral 

 surface of the thorax and its appendages, while a smaller branch 

 running backwards also beneath the nerve -chain is the inferior 

 abdominal artery (absent only in Paguridea). These two arteries 

 taken together form a median longitudinal trunk quite comparable 

 to the subneural vessel of Isopods, and, like it, may communicate 

 with the dorsal system of vessels by a circumoesophageal ring. 

 A further communication is very often present at the posterior end 

 of the abdomen, where a vascular ring encircling the intestine 

 unites the superior and inferior abdominal arteries. A pair of 

 posterior lateral arteries arising from the superior abdominal 

 artery near its origin from the heart, and often unsymmetrically 

 developed, are of importance since they irrigate the branchiostegal 

 regions of the carapace which have a respiratory function. 



A venous sinus in the mid-ventral line receives the blood from 

 the lacunar system of the body and appendages and distributes 

 it to the gills, whence it is returned to the pericardial sinus by 

 branchio-pericardial channels running in the inner wall of the 

 branchial cavity. A minor circuit for the blood is afforded by the 

 lacunar network of the branchiostegites, which, receiving blood 

 partly from arteries and partly from adjacent venous sinuses, 

 return it directly to the pericardium by special channels. 



In terrestrial Decapods various modifications of the respiratory 

 and circulatory systems are met with. In those most completely 

 adapted to a terrestrial life (Birgus, Cardisoma) the lining membrane 

 of the branchial cavity is very vascular and covered with minute 

 villi. The supply of venous blood to the sinuses of the branchio- 

 stegal regions is more important and more definite than in 

 aquatic Decapods, and the apparatus no doubt functions as a 

 lung. , In the terrestrial Hermit-crabs (Coenobita) a very peculiar 

 respiratory organ is found. A rich vascular network is developed 



