174 
ANODONTA — CIRCULATORY SYSTEM. 
ventricle by sinichrouous contractions, regurgitation being prevented 
by the large pocket-shaped daps at the i)oint of junction, distinguished 
as the anri-ventricnlar valves. 
The ventricle, which receives the blood from the anricles, is a 
medially placed vessel, of an elongate and pyriform shape, with the 
thick and somewhat bilolied end directed posteriorly ; it has thick, 
spongy and muscular walls formed of interlaced muscle dbres, and 
gives origin to a large arterial trunk at each end, distingui.shed in 
accordance with their re.spective positions as the anterior or the 
posterior aorta, which transmit to the body the arterial blood expelled 
from the ventricle by the strong and successive waves of contraction 
which ]iass rhythmically from one end of the ve.ssel to the other. 
Fig. 342. — Anodonta cygnca^ injecteil from the ventricle to show the arterial system. The right 
pallial lobe, gills and mcsosoma partially removed and pericardium opened (after Howes). 
a. auricle of heart : a. a. anterior aortic huUi ; a.s. anal siphon ; n.ad. anterior adductor ; a. pi. a 
anterior pallial artery ; hr. ))ranchi;u or gills ; hr.s. branchial siphon ; d.o. dorsal orifice ; /. foot ; 
/. liver or digestive gland; La. labial artery; l.p.a. left posterior artery; /. pedal artery: 
p.a. origin of posterior aorta ; p.nd. posterior adductor ; p.p/.a. posterior pallial artery ; r.p.a. right 
posterior artery ; v. ventricle of heart ; "'.a. visceral artery, 
'I’he untcuiur aorta runs forward above the intestine, which it 
])artially encloses, Iweaking up into various arteries, the two princij)al 
being tbe visceral artery, which sup})lies the intestines, the digestive 
and genital glands, and the pedal artery, which passes through the 
cerebro-jdeunj-pedal nerve loop and sup})lies the foot, which is rendered 
turgid and protrusible by blood pressure, the revulsion or reflux of the 
contained Wood towards the ventricle, which takes place upon any 
sudden contracti(jn or withdrawal of the foot being arrested by a 
valvular arrangement in connection with the anterior aortic bulb ; 
while the anterior pallial artery su})plies the oral lobes and spreads out 
over the anterior part of the mantle and unites within the mantle 
margin with the ])osterior pallial arteries. 
