ANODONTA — CIRCULATORY SYSTEM. 
175 
The posterior aorta is also furnished udth a valvular arrangement 
preventing the reflux of the blood to the ventricle, and has a backward 
course beneath the rectum, soon dividing into two large lateral 
trunks, the right and left posterior pallia! arteries, whose principal 
Fig. 3t3. — Transverse and somewhat dia- 
grammatic section through the middle peri- 
cardial region of Anodonta cy^nea, after 
injection, to illustrate the circulation within 
the branchiae, renal organ, pericardial gland, 
and pallial lobes, and also showing the open 
communication between the infra-branchial 
chamber and the supra-branchial cavities of 
the inner gills (after Howes). The vessels 
containing venous blood are black, except the 
efferent pedal veins, and the arrows indicate 
the direction of the blood currents. 
a, auricle of the heart ; a.v. afferent 
branchial trunk ; c.7'. efferent branchial and 
pallial trunk ; /. foot, showing the efiferent 
pedal v'ein and its junction with the vena-cava; 
ii'.c. lower or infra-branchial chamber ; i.c. 
inner ctenidium or branchia ; ;//. mantle lobes, 
showing pallial blood vessels, pb.v . ; o.c. outer 
ctenidium or branchia ; pc. pericardium ; 
pc.g» pericardial gland, showing capillary 
plexus ; r. rectum, showing typhlosole ; r.pi. 
renal capillary plexus ; sb.c. supra-branchial 
chamber ; v. ventricle of the heart v.c, 
vena-cava, or central venous blood sinus. 
branches run along the free edge of the mantle, anastomosing therein 
with the anterior pallial vessels, hut which early give off smaller 
vessels, supplying the rectum, the pericardium, the posterior adductor, 
the siidional retractors, and neighbouring tissues. 
Fig. 344. — Anodonta cygnea, injected to show the larger vessels in connection with the venous 
system, especially in relation to the plexus within the renal organs. The right mantle lobe 
removed and the external gill lamina, pericardium and ventricle opened up (after Howes). 
a. auricle of the heart ; a. ad. anterior adductor ; a.p. anterior protractor muscle ; af.br.v. afferent 
branchial veins distributing blood within the gills ; ef.p.v. efferent pedal veins, continuous above 
the nephridia with the vena-cava, the dotted line approximately indicating the position of 
Kebers valvule ; /. foot ; p.ad. posterior adductor ; pc. pericardial chamber ; r. rectum, passing 
through ventricle ; r.pl. renal capillary plexus ; v. ventricle of heart opened up to show the junction 
of the auricle and the relations of the rectum. 
After permeating the body, the blood eventually reaches the venous 
sinuses, of which the principal are the pallial, the pedal and the great 
longitudinal central blood vessel or Vena-cava, which represents the 
circular jnilmonary sinus of JleU.v, and lies between the pericardium 
