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cniCUr-ATiiRY SYSTEM -AORTA. 
The jMonotocardi.vte (/ioro9, single ; wrn, auricles ; KupSiu, heart) 
heart more esi)ecially characterizes the Piilmoiiata, iii M'hich the organ 
is larger than in the Azygobranchiate Streptuneura, M’hich also possess 
it; although the most simple form, it is a simplicity derived from the 
more complicated Diotucardiate organ, and consists of a single 
Fig. 579. Fig. 580. Fig. 581. Fig. 582. 
Diagrams showing ihe character of the heart in various groups of moliusca and its structural 
relationship to the more primitive organization of the Anneliils (after Gegenbaur). 
Fig. 579.— Fart of the dorsal propulsory vessel of ventricle of a worm with some of the lateral 
contributory vessels or auricles. 
Fig. OvSO. — Heart and paired auricles of jY<j7(ti7u$ showing the reduction to two pairs of auricles, 
corresponding to the four branchiaj. 
Fk;. 581. — I )iotocardiate heart of a Felecypod, showing the further reduction of the lateral 
auricles to a single pair in correlation with the paired branchta;. 
Fu;. 582. — Monotocardiale heart of a Gastropod, showing the single auricle, its fellow having 
disappeared in conformity with the loss of the primitively left branchia. 
a. auricle ; f. ventricle ; a. a. anterior aorta ; /.a. posterior aorta. 
ventricle :ind a single auricle, the anrimdar reduction being correlated 
with the loss of the left branchia' and also with the presence of the 
single pulmonary chamber. 
'I'he Circulatory vessels are usually divided into an arterial and 
a venous system. The Arterial system, in addition to the heart, is 
constituted by the aorta*, tlie arteries, the arterial lacumT, and the 
capillaries, all conveying the blood centrifugally, or from the heart to 
the various organs of the body. The Venous .system is composed of 
tubular ve.s.sels and a limited number of spacious cavities or venous 
siuu.ses, the encloseil venous blood moving centripetally or from the 
organs and e.xterior of the l)ody towards the heart, becoming accumu- 
lated within the venous sinuses Ijefore entering the respiratory organs. 
'I'he Aorta (do/iTedg a. lengthened form of dei/)(o, to raise or hang 
u}), so called because it was sipiposed to hang up and keep in position 
the vertebrate heart, as indeed to some extent it does) is the main 
trunk of the arterial system in the mollu.sca and is usually short, 
originating at the ventricle and directly receiving the blood from it 
for distribution through the body. In the Pelecypoda there is usually 
an anterior and a posterior aorta, which arise from opposite ends of 
the longitudinally placed ventricle, Imt in (Uir Gastropods the repre- 
