366 COMPARATIVE ANATOMY chap. 



segment a pair of lateral arteries which supply the intestine, integu- 

 ment, and musculature of the abdomen. The sub-neural vessel which 

 receives its blood from the sternal artery, is already known to us in 

 the Isopoda and Stomatopoda, here, however, and in all Decapoda it plays 

 a much larger part, since lateral branches from it supply all the limbs 

 from the maxilliB to the last pleopoda. It also serves for nourishing 

 the ventral chord, this being its almost exclusive function in the 

 Stomatopoda, the limbs there being more often supplied by branches of 

 the lateral arteries of the heart. All the arteries branch richly and 

 pass into arterial capillaries, which open into the venous lacunar 

 system of the body. Even though the flow of blood in this lacunar 

 system is, as in all Crustaceans, regular and constant, and though 

 venous blood canals often come into existence, we do not find among 

 the Decapoda any veins with walls of their own. The system of venous 

 cavities, taken as a whole, represents the body cavity. We can only 

 give a few details as to the course of the venous blood. It nearly all 

 flows together into a large ventral blood sinus in the cephalothorax, 

 from the lateral parts of which canals conduct it into the gills, 

 while other canals convey the blood which has become arterial in 

 the gills away from them to the pericardial sinus. The respiratory 

 organs are therefore here placed in that part of the circulatory 

 system which conveys the blood out of the body back to the heart, 

 and this is the ease in all Arthropoda. The arterial blood coming 

 from the gills leaves the pericardium, and, mixed with the blood 

 which flows back out of the cephalothoraoic shield, enters the heart 

 through its ostia. The ostia as well as the points of origin of the 

 arteries of the heart are provided with valves. The valves of the 

 former prevent the return of the blood into the pericardium, those of 

 the latter its return out of the arteries into the heart. When the 

 heart contracts (systole) the blood contained in it is driven into the 

 arteries, and when it again expands (diastole) it sucks in blood out of 

 the pericardium through the ostia. 



In the Mysidce (with the exception of Eujihausia) there are 2 or 3 unpaired 

 hepatic arteries, springing from tlie ventral wall of the heart. The abdominal portion 

 of the sub-neural vessel is wanting. In the male of Siriella, which carries gills on 

 its pleopoda, the latter receive their blood from branches of the lateral arteries of 

 the posterior aorta. In the Schisopoda, Cmnacca, and larvce of Decapoda in which 

 the gills are not yet developed, the blood circulation in the cephalothoraoic shield 

 with its fold is very brisk, and there is no doubt that in those forms which have no 

 special gills the respiration principally takes place in it. In Siriella and Mysis and 

 perhaps also in other Schizopoda it is most probable that the integument forming the 

 inner wall of the respiratory cavity has also a respiratory function (comp. Fig. 222, 

 p. 321). The channels which convey the blood out of the thoracic feet back to the 

 heart cause ridge-like projections in this integument which may be called branchial 

 ridges. The constant vibrations in the respiratory cavity of the epipodial appendages 

 of the first thoracic foot create a constant exchange of water in it. 



We have already described (p. 329) the respiratory organ of the air-breathing 

 Birgus latro. AVe will here briefly describe the circulation of the blood in connection 



