CIRCULATORY SYSTEM — AURICLE. 
291 
united in tlie median line to form a common propulsive vessel, en- 
closing the rectum between their walls. 
The Auricles {nuricul't, diminutive of aiiris, the ear ; the auricles 
in man being said to resemble 
the external ears of some (quad- 
rupeds) are generally pyriform, 
thin and transparent, with few 
muscle-fibres, usually innervated 
by the pallial ganglia, and often 
smaller than the Ventricle, 
especially in the Pulmonata ; 
they receive the blood direct 
from the respiratory organs 
and discharge it into the vent- 
ricle by a slender neck, its 
reflux being prevented by the 
auri-ventricular valves at the junction. Valvular contrivances may 
also be qiresent in various other qiarts of the system, more esjiecially 
in coimection with the dittereiit turgescible organs, as the foot, 
tentacles, siqilioiis, etc. The number of auricles varies in correlation 
with the number of respiratory organs, the Pelecypoda always 
qiossessing two symmetrical auri- 
cles, corresqionding to their qiaired 
brauchiie, but in our Gastropoda 
they are reduced to one only, owing 
to the loss of the primitively left 
branchia. The contractions and 
dilations of the auricles, when 
paii’ed, ai’e always simultaneous, 
and in every case alternate with those of the ventricle. 
The number of auricular chambers to the heart has formed a 
basis for the grouping of the mollusca in two sections, Diotocardia 
and Monotocardia, which, like other structural details, are also of 
value as showing phylogenetic relationship. 
The Diotocaruiate (Siw, two ; (oTa, auricles ; KapSia, heart) heart 
is characteristic of the Pelecypoda and Zygobranchiate Streptoneura, 
and is composed of a pair of symmetrically disposed lateral auricles 
and a single ventricle, which is often pierced by the rectum, and 
though the most complex is the more qu’imitive form. 
Fig. 578. — Transverse section through the 
heart of Anodonta cygnea (L ), X 2 (after 
Rankin), showing the structure of the walls 
of the ventricle, the arrangement of the auri- 
cular valves and the position of the rectum. 
an, auricles ; r. rectum ; ty. typhlosole. 
Fig. 577. — Heart of Patella with the ventricle 
laid open, to show the muscular pillars or 
colummp cariKF and the valvular opening.'^ to the 
auricle and to the arterial system, greatly en- 
larged (after Wegmann). 
V, interior of ventricle with the vertical 
cohtmme carno’ and showing the valve of the 
auricle to the left and the aortic valve towards the 
base ; b.iK branchial vein, also showing the 
openings through which the blood from roof of 
pallial cav’ity enters the auricle ; auricle ; a. a. 
anterior aorta ; p,a. posterior aorta ; a.h. aortic 
bulb. 
