294 
CIRCULATORY SYSTEM — VEINS, ETC. 
sinaller arteries and tlie hegiimiiig of the smaller veins ; these inter- 
mediary vessels which cunneet together the arterial and the venous 
sy.stems are distinctly developed in TestaceUd and other species. 
The Veins {vena, blood vessel) are the blood channels which convey 
the blood back to the heart, whether by way of the respiratory organs 
or direct ; they are continuous with and receive the blood from the 
capillaries, hut gradually unite to form larger vessels, which 
eventually empty into the large venous sinuses iireparatory to 
diffusion in the respiratory organs. 
Tlie Venous Sinuses, or blood reservoirs, are large cavities within 
which the blood, exhausted of its oxygen and loaded with the waste 
I'Ki. oSx— I )iagrain of the circuialory .'^ysicin of especially illustrating the venous sinuses 
(after GritVilijs). 
7'..^. venous sinus ; r.7'..Y. circular jmlmonary sinus ; a. aurie'e, receiving the liloocl after aeration 
in the pulmonary ple.xus ; 7-. \ eniricle, giving olT the anterior and posterior aorlat. 
matters from the oxidation of the tissues, is accumulated after its 
circuit of the body, prior to entering the renal and resi)iratory organs 
for purification. 
In the (lastropoda the chief venous sinuses are in the pedal and 
]iallio-visceral regions, while in the I’elecypoda the chief venous 
sinus is known as the Vena-cava, and is a spacious cavity lying 
longitudinally beneath the iiericardium. 
'I'he Blook or Iheniolymph blood ; Jifinpha, water) is the 
circulatory fluid, and is a slightly viscous semi-transparent albuminoid 
tlnid, remarkable for the (piantity of calcic carbonate it contains, as 
shown by its effcrve.scence wlien treated with acids. It supplies the 
vaihnis glands with the material from which they elaborate their 
various secretions and also receives the iirodiicts of the oxidisation of 
the various tissues, conveying them to the excretory organs for 
