120 CRUSTACEANS. 



the abdomen, while the other turns downward toward the 

 ventral surface. All these arteries convey blood out from 

 the heart directly to the tissues, through their many 

 minute branches. The blood has no such channels for 

 return, but finds its way back to the gills by percolating 

 through the interspaces of the tissues and internal organs. 

 It comes back, necessarily, laden with carbonic-acid gas, 

 the product of oxidation in the tissues, and must needs 

 go to the gills to exchange this noxious gas for a fresh 

 supply of oxygen. There is a passageway for the blood 

 up one side of the central supporting portion of the gill, 

 and down the other ; and to get from one into the other, 

 the blood must flow through very delicate capillary tubes 

 that lie out in the most delicate gill filaments, and connect 

 the two passages. In passing through these capillary 

 tubes, the blood currents on the inside of them are sepa- 

 rated from the water currents outside only by very thin 

 membrane, through which the necessary exchange of gases 

 takes place. The blood is then returned from the gills 

 through the vertical canals already noticed in the thoracic 

 wall, to the pericardial sinus. It reenters the heart 

 through six apertures which are guarded by valves iii- 

 side. One of these apertures may be seen on each side 

 of the heart, a pair on its upper surface, and a pair on 

 its lower surface. Find them, using a lens. When the 

 heart is filled with blood, it contracts ; and the blood, 

 being prevented by valves from flowing back into the 

 pericardial sinus, is forced out through the arteries. 

 When the heart relaxes again, the valves fly open, and 

 the blood from the sinus rushes in and fills it, and is 

 driven outward through the arteries at its next contrac- 

 tion. The currents coming up from the gills continually 

 renew the supply in the pericardial sinus. 



Several delicate fibrous bands may be seen connecting 

 the heart with the walls of the pericardial sinus. 



