26 The Shell-Collector’s Handbook. 
seems to be the production of a larger absorptive 
surface. 
The liver or digestive gland lies on the sides of the 
stomach, into which it opens by several bile ducts. 
It is multi-lobed, and of a brownish colour. 
THE Giiis.—The gills lie between the visceral mass 
and the mantle. Each gill consists of a pair of 
lamelle, and each lamella is composed of a large 
number of gill filaments which have become fused 
together so as to form a perforated trellis-work-like 
structure. The blood circulates through these gill 
filaments, which thus form a kind of ciliated grating 
through and over which the water is driven. 
THE CrrcuLatory Oreans.—The heart consists of 
two lateral auricles and one median ventricle, and is 
situated just under the hinge-line and above and 
behind the digestive gland. The pericardial cavity 
is pierced by the intestine. 
From the anterior end of the ventricle the anterior 
aorta arises, and after entering the visceral mass 
divides into visceral and pedal arteries, which supply 
the anterior two-thirds of the body, including the 
foot, labial palps, anterior part of the mantle, and 
intestine. From the posterior end of the ventricle, 
the posterior aorta arises, and, after dividing into a 
right and a left branch, supplies the remaining portion 
of the body. 
These vessels, as in the Gasteropods, break up into 
lacunar spaces, from whence the blood, after giving 
nutriment to the tissues, finds its way into a large 
sinus situated under the pericardium, and known as 
the vena cava. From this it is carried to the organ 
