CIRCULATORY SYSTEM — ARTERIES, ETC. 
298 
seutatives of the anterior and po.sterior aortie of the Pelecypoda 
combine before reaching tlie ventricle, to form a single aortic vessel, 
their approximation and basal fusion being probalily due to the body 
torsion the Gastropoda have undergone. 
The Arteries (ai/p, air ; T>/p«o, to keep ; the ancients believing 
these vessels naturally contained air) originate by the furcation of the 
aortic and are of smaller calibre. In the Gastropoda one branch, 
dividing and redividing, traverses the body and supplies the intestinal 
tract, the digestive gland, the genital gland, etc., and is known as the 
Visceral artery, while the other, known as the Cephalic artery, bends 
round and supplies the head and its organs, encircling the hrst 
Fig. .583. — Tcstacella halhtidca Drap. laid open dorsally and the viscera partially removed to 
show the arterial blood vessels, x 2 (after Lacaze-Duthiers). 
st. stomach ; r.s. radular sac ; a. auricle of the heart ; ventricle, from which originate the 
anterior and posterior aortae and the subsidiary arterial vessels. 
anterior intestinal coil and sending a strung branch to the foot. In 
some species, and especially in ^-1 riun (iter, the walls of the arteries 
and arterioles are greatly thickened by a siUTOuuding sheath of con- 
nective tissue, containing numerous calcareous and fatty particles, 
which gives the vessels an opm^ue, milk-white appearance, beautifully 
contrasting with the dark tissues upon which their complex ramihca- 
tions are traced. 
The arterial vessels may terminate and be joined to the venous 
system by finer and more delicate vessels or may 0 })en by wide 
funnel-shaped and sometimes contractile orifices into laciime. 
The L.\cun.e (Jiicuna, a pool), Ihema- 
tocades or arterial blood sinuses, are 
the intervening spaces which permeate 
between the various organs of the body ; 
they vary greatly in size and capacity, 
and are destitute of proper endothelial 
covering. 
The Capill.vries (ccipUki, a hair) are the minute, thin-walled blood 
vessels which, in some forms, constitute the terminations of the 
Fig. 58i.— Connective ti>.sue cel s 
of virion ntcr (L ), showing the 
origin of lacunre, highly magnified 
(after Brock). 
