EYDER.; 



LIFE-HISTORY OF THE OYSTER. 323 



These values at the lower end of the ventricle open upwards, so that 

 we may confidently Infer that the blood of the oyster flows constantly 

 111 one direction, or from the auricles through the ventricle and from 

 thence through the great posterior and anterior arteries a and a> to be 

 distributed to the opposite ends of the body. 



The hindermost artery a carries fresh blood mainly to the great mus- 

 cle M, while the anterior one «' carries blood to the body-mass ant^ 

 riorly. After entering the body-mass at a' the anterior artery imme- 

 diately divides and sends a dorsal branch «' forward, and a ventral 

 branch a" downward and forward; these two vessels are shown cut 

 across in the section represented in Fig. 3. The two main anterior ar- 

 terial twigs give off many small branches at intervals which traverse 

 the soft substance of the body, but in some portions there seem to be 

 no true vessels, but rather irregular vascular spaces which in all proba- 

 bility communicate with the vessels just described. The tissue in which 

 we find more or less evidence of the existence of irregular blood- 

 spaces, IS that indicated by the letter c in Fig 3, and is the tissue which 

 envelopes all of the internal organs, extending even into the man- 

 tle and gills. This common supporting or connective tissue forms 

 the walls ot all the great arteries and veins throughout most of 

 their extent, especially where these traverse the soft body-mass, palps 

 and giUs. A few of the vessels have proper membranous walls 

 such as the branchio-cardiac vessels hr, Fig. 1, which bring the blood 

 back to the heart from the gills, mantle, and renal organs. 



At the lower side of the body a large vein, the vena cava, w. Fig 3 

 receives the blood from the upper and anterior part of the body to con' 

 vey It to the gills to be oxygenated before it is returned to the heart 

 again. How the blood sent from the heart to the hinder part of the body 

 IS carried to the gills and back to the heart, the writer has not been 

 able to make out clearly. There is also a system of vascular channels 

 which traverse the adductor. 



_ The arteries of the palps are superficial, and are shown in ¥W i but 

 m the deeper, fleshy portions of the palps, definite vascular channels 

 are replaced by irregular vascular spaces. Injections and sections also 

 show that there are definite vascular channels in the mantle and gills 

 and in the former, especiaUy when the oyster is very emaciated these 

 sometimes have very thick walls. 



To sum up what we have stated regarding the course of the blood 

 we find that it passes from the gills to the heart, thence to tbe various 

 parts of the body, and then directly to the gills again. It will be noticed 

 that this is an arrangement very diflerent from that found to obtain in 

 fishes, where tbe heart receives the blood from all parts of the body 

 sends It through the gills, and then on directly to the various parts of 

 the body. The difference between the circulation of the oyster and that 

 of a warm-blooded, air-breathing animal with a four-chambered heart is 

 stm greater, from the fact that in such forms the heart receives the 



