BLOOD SYSTEM. 4Q] 



blood is gathered into large reservoirs (blood sinuses), 

 one on each side, running along near the base of the 

 limbs. From these sinuses the blood is again taken up 

 by the veins, to be distributed to the gills (Fig. 278) and 

 oxidized, and thence to the heart, to be again distributed 

 to the tissues. Fig. 279 is a schematic diagram showing 

 the general course. 



It is plain that the heart in this case is physiologic- 

 ally a systemic heart, since it contains only oxidized blood, 

 and throws it to the tissues. 



MOLLUSCA. 



Acephala, or Bivalves. — Take as an example the 

 river clam {Anodonta). Fig. 280 is a longitudinal, and 

 Fig. 281 is a transverse section. The gills, two on each 



Fig. 280.— Vertical longitudinal section of Anodonta : pm, am, posterior- 

 anterior adductor muscles ; m, mouth ; st, stomach ; cce, caecum ; H, 

 heart ; /, foot ; G, gills ; ma, mantle. 



side, are cellulated sacs, very much divided, to produce 

 large surface of contact with the aerated water. 



Breathing, — Ciliary currents pass from behind for- 

 ward through the gills, determining oxidation of the 

 blood, and thenceforward to the mouth for alimenta- 

 tion, as already explained (page 338), and then backward 



