MOLLUSGA-^THE CIRCULATORY SYSTEM 



20: 



ventricle, but arc drawn down to its lower side, where they grow together, communi- 

 cating through a more or less large aperture. Pinna, Avicnla, and Perna exhibit 

 the consecutive stages in the displacement of the heart to the lower side of the hind- 

 gut. The shifting of the gills from the original region of the heart just mentioned 

 is caused by the shifting forward of the posterior adductor, which grows more and 

 more massive and finally reaches a median position on the shell valve. It has already 

 been mentioned that this posterior adductor, by the continuous reduction and final 

 disappearance of the anterior adductor, becomes the one adductor of the Mono- 

 myaria. 



In Teredo also, the heart lies on the under side of the hind-gut. This is con- 

 nected with the approximation of the hind-gut with the anus to the mouth dorsally, 



Fig. 172. 



Fig. 173. 



Fig. 172.— Transverse section through Anodonta, to illustrate the course of the circulation 

 of the gills and the kidneys, and the branchial veins (after Howes), hr. Gills ; hre, efl'erent 

 branchial vessel (branchial vein) which opens into the large branchi^ vein hrei, running along the 

 base of the gills, and here cut through transversely ; pv, pallial vein ; vc, large venous sinus of the 

 body;; kb, pericardial gland ; owj, auricle ; rx, rectum ; v, ventricle ; rv and rvy, renal vessels ; hrai, 

 afferent branchial vessel (branchial artery), running"along the base of the gills ; hm, lateral branches 

 of the same running in the gills. The veins or sinuses conveying venous blood are black. 



Fig. 173.— Another section through Anodonta (after Howes). Lettering as in Fig. 172. au, 

 auricle ; sbc, spaces at the base of the gills, bathed by the water and communicating with the 

 mantle cavity, between the ascending and descending branchial lamellse. 



while the gills, remaining in their original position, retain the heart on the lower 

 side of the hind-gut. 



Circulation (Fig. 2.5, p. 17). — The arteries have walls of their own, and branch 

 into fine vessels, which discharge the blood into the lacunar system of the body. 

 The venous system seems to have no distinct vessels with walls of their own. 

 although it forms more or less wide channels resembling true vessels. 



An anterior ami a posterior aorta spring, as a rule, from the ventricle. The 

 anterior aorta runs forward above the intestine and breaks up into various arteries. 

 The arteria visceralis supplies the intestine, the digestive gland, and the genital 

 gland ; the pedal artery supplies the foot ; the anterior pallial artery spreads out 

 over the anterior pait of the mantle and the oral lobes (labial palps). 



