Feeding and Breathing 47 



gills the oxygenated blood flows to the heart, whence 

 by way of the aorta it is distributed throughout the 

 body, and returning by veins it is again passed 

 through the gills. It must not be supposed, however, 

 that the circulation is as rigidly confined in definite 

 tubular vessels as in the higher animals. Much of 

 the blood appears to flow indefinitely in irregular 

 spaces of the body called laciinoi and sinuses, getting 

 reinforced by that which has passed through the gills 

 and heart, but perhaps not all of it finding its way 

 back to these organs. The vital fluid is in most 

 mollusks colourless, but a few of the more active 

 species possess the red colouring matter (hce'moglobin) 

 which distinguishes the blood of vertebrates. 



