364 COMPARATIVE ANATOMY CHAP. 



responds with the heart of the Decapoda. It reaches to the 

 posterior limit of the first maxillipedal segment, possesses a large 

 pair of ostia, and gives off the following vessels : (a) an anterior un- 

 paired cephalic aorta (ac), (b) and (c) an anterior weaker and a posterior 

 stronger pair of arteries. The second division, the many-chambered 

 dorsal vessel, possesses 12 pairs of ostia, and gives off 13 pairs of 

 arteries and an unpaired posterior aorta. To complete the whole picture 

 of the circulatory system and to show its relation to that of the Isopoda, 

 we must add that a median subneural vessel runs under the ventral 

 chord through the whole body, that the whole venous system is 

 laeunar, and that there are two principal venous sinuses, one ventral, 

 and the other dorsal. The arterial system, on the other hand, is 

 composed of richly-branched vessels having Avails of their own and 

 breaking up into capillaries. 



As to the more detailed arrangements, the cephalic aorta supplies the eyes, the 

 two pairs of antennte, the brain, and the anterior lateral regions of the shell. The 

 most anterior pair of arteries supplies the mandibles and maxillse, and the central 

 part of the shell. The large 2d pair of arteries probably supplies the maxilla? and 

 maxillipedes ; one artery passes between the longitudinal commissures of the ganglia 

 of the 1st and 2d maxillipedal segments to connect itself with the subneural vessel 

 (compare the sternal artery of the Schizopoda and Decapoda]. The subneural 

 vessel gives off primarily vascular loops to the ganglia of the ventral chord, but it 

 also gives off branches to the limbs. The 13 pairs of arteries of the many-chambered 

 dorsal vessel supply the thorax and abdomen with their extremities in such a way 

 that the pair of arteries belonging to a pair of ostia spread out, not in the segment 

 in which the ostia occur, but in that immediately in front of it. The whole heart 

 seems to have been shifted back one segment, so that the pair of ostia lying in the 

 first abdominal segment (there are 7 pairs of ostia in the abdomen) originally belonged 

 to the most posterior thoracic segment. The posterior aorta richly supplies the 

 telson with lateral branches. In the venous system paired lateral blood sinuses 

 conduct the blood out of the extremities and other organs into the large ventral 

 sinus. The blood streams thence [into the pericardial sinus and throxigh the ostia 

 back into the heart. It is only in the abdomen, whose limbs carry the gills, that 

 the blood which has become arterial appears to flow direct back into the pericardial 

 sinus, avoiding the ventral sinus. 



A comparison with younger Squilla larvre of the so-called Erichthoid stage makes 

 it highly probable that 2 pairs of anterior ostia of the dorsal vessel there present 

 disappear in the course of development. While, as a rule, in the many-chambered 

 dorsal vessel a pair of ostia lies over each outgoing pair of arteries, there are no 

 ostia to correspond with the two most anterior pair. 



Among the Thoracostraca a chambered dorsal vessel provided with many pairs of 

 ostia and reaching into the abdomen is only found in the Stomutopoda. This more 

 primitive condition has here been retained, evidently in connection with the localisa- 

 tion of the respiration in the branchial tufts of the abdominal limbs. 



The blood-vascular systems of all the other Thoracostraca, at least 

 of the Schizopoda and Decapoda, closely resemble one another, and must 

 be contrasted with that of the Stomatopoda. [The blood-vascular system 

 of the Cumacea is not yet thoroughly investigated ; it probably agrees 

 to a great extent with that of the Schizopoda and Decapoda.] 



