THE MYSIDACEA 



177 



FIG. 107. 

 Alimentary canal of Mysis. 



st, cardiac chamber of stomach. 

 (After Sars.) 



floor of the pyloric division, and bearing on each side anteriorly a 



double comb-like row of iridescent setae, can be identified with a 



similar process found in Amphipoda and Isopoda. The extent of 



the mid-gut does not seem to have been 



determined. Five pairs of hepatic caeca 



(Fig. 107, hep) are found in Mysis, opening 



by a common duct on each side just behind 



the tongue-shaped process on the floor of the 



pyloric chamber. Two pairs are very short 



and directed forwards ; of the three pairs 



directed backwards, the upper and lower 



extend through the greater part of the 



thorax while the middle pair are much 



shorter. In Siriella there are only three 



pairs, one turned forwards and two longer 



pairs turned backwards. An unpaired dorsal 



diverticulum (d) is given off at the junction 



of stomach and intestine. 



Circulatory System. The heart of the 

 Mysidae is elongated, fusiform, extending 

 the whole or the greater part of the length 

 of the thoracic region. There are only two 

 pairs of ostia, one dorsal to and slightly in advance of the other. 

 Anteriorly and posteriorly the heart is continued into median aortic 

 vessels each flanked at its origin by a pair of lateral vessels. From 

 the under-side of the heart a number of median vessels are given 

 off to the underlying viscera, and near the posterior end there 

 originates an unpaired descending artery which passes on one side 

 of the intestine. On approaching the sternal surface it divides in 

 the median plane into three branches which pass between the con- 

 nectives of the nerve-chain in the fifth, sixth, and seventh thoracic 

 somites. The anterior branch is continued forwards as a subneural 

 artery through the anterior thoracic somites ; the- middle branch 

 supplies the sixth and the posterior the seventh and eighth somites 

 and their limbs. The abdominal aorta gives off in each somite, 

 besides paired vessels which terminate in the pleopods, a median 

 branch which passes on one side of the intestine and runs for a 

 little way alongside the ventral nerve-cord, sometimes anastomosing 

 with its neighbours in front and behind. The interest of this dis- 

 position of the arterial trunks lies in the fact that the descending 

 artery given off from the posterior end of the heart, which is clearly 

 homologous with the similar vessel found in the Decapoda, is here 

 seen to be one of a series of median vessels originating from the 

 under-side of the heart and of the abdominal aorta, and contribut- 

 ing to the formation of a discontinuous sternal or subneural vessel 

 on the ventral side of the body. 



