MOLLUSC A. 



25 



The blood, after its passage through the fine branches 

 of the arterial system, is passed on into still finer vessels, 

 which form masses of spongy-looking tissue. These mi- 

 nute tubes, or capillaries as they are called, unite into 

 larger tubes, and lead the blood into the larger veins. 



To illustrate this, imagine that you had two bushes, 

 each with numerous branches, but the stems hollowed out 

 into tubes of different sizes opening at the ends of the 

 twigs. 



Then that there was a great mass of tubes smaller than 

 the terminal twigs of the two bushes, all of these so inex- 

 tricably interlaced, and communicating with each other by 

 means of intercepting tubes, that the whole mass would 

 look like a great sponge. One bush of hollow tubes could 

 be made to represent arteries ; and by connecting the hol- 

 low twigs of the former with the tubes of the exterior of 

 the spongy mass, fluid could be forced through the stem 

 into the twigs and fill the sponge. The other could be 

 considered as representing the venous system ; and its 

 twigs being connected with the spongy mass, on the side 

 opposite to the arteries, would receive the blood as it 

 flowed through the fine network of vessels in the sponge, 

 and collect it into larger and larger tubes until it reached 

 the stems of the venous system. If now the stems, both 

 venous and arterial, were supposed to be dilated into bags 

 at their largest ends or roots, and these connected together, 

 the arterial bag with the venous, so that the blood would 

 pass from the bag at the end of the venous stem, back into 

 the bag at the beginning of the arterial stem, and com- 

 mence again to run towards the spongy connecting mass, 

 this would represent the relations of the arteries and veins 

 in the mantle of the oyster, but not in the other parts of 

 the body. 



Instead of passing from the spongy masses into the 

 smaller veins, and thence to the auricles, the larger 



