13 



its walls are made up of a delicate meshwork of muscu- 

 lar fibers so interlaced as to be altogether untraceable ; 

 by its contractions it forces the oxygenated blood fresh 

 from the gills to all parts of the body through two large 

 vessels, or properly, aortae, the first, ao^ supplying the 

 forepart of the body, and ao' the hinder portion. These 

 larger vessels divide into smaller ones which effect the 

 distribution of the blood towards the head, and through 

 the viscera, where, as in other parts of the body, a net- 

 work of fine capillary vessels exists. The blood accumu- 

 lates perhaps in sinuses, one of which is the pericardial 

 space, and there are besides probably a pair of rudimen- 

 tary ones in the oyster, the vestibule of a rudimentary 

 organ of Bojanus, from these, through the venous sys- 

 tem of vessels, the blood is carried back to the gills to 

 be aerated, and returned through the vessels which carry 

 the blood from the gills to the auricles, the branchio- 

 cardiac vessels he. Of the last, there are three on each 

 side of the oyster, two pairs of which collect the blood 

 from the forepart of the gills, and one pair of which 

 gather the blood from the posteriar portion of the gills 

 to be carried to the auricles, from whence it enters the 

 ventricle, having completed its circulation through the 

 whole vascular system. 



It is frequently noticed that the edges or a consider- 

 able part of the mantle of the oyster is very much swol- 

 len and nearly transparent, as though greatlj^ distended 

 with water ; this is due to a peculiar arrangement of ca- 

 pillary vessels connected with the blood-system which 

 causes the blood to be held in these places until turges- 

 cence is produced as in erectile tissues. The presence 

 of an unusual amount of Water in the blood no doubt 

 sometimes helps to produce this last effect, since the 

 blood- system is probably in communication with the 

 water outside, throiigh the openings from without into 

 the organ of Bojanus, a part which answers to the kid- 

 ney in other bivalve mollusks, but which seems to be 



