150 MOLLUSCA.« 
obsolete. The eyes are two in number, in the form of black 
points, seated at the tips of the posterior tentacula. 
In some of the genera the cloak is furnished with a shield, 
which is, in general, strengthened internally by a deposition 
of earthy matter, in the form of grains, or a shelly plate. 
The shield in several of the genera is placed anteriorly, 
or the shield is placed nearer the head than the tail. The 
group thus distinguished contains four genera, two of which 
have compound. tails, or furnished with peculiar organs, 
while in the remaining genera the tails are simple. The 
mouth consists of lips, which are capable of small exten- 
sion, and above, the entrance is armed with a concave cor- 
neous jaw, with a notch in the middle. The tongue is merely 
armed with soft transverse ridges, pointed before, and ter- 
minated by a short cartilaginous cone. There is a sensible 
dilatation of the gullet, which marks the place of the sto- 
mach, at the under extremity of which is the rudiment of 
a cecum at the pyloric opening. The intestine makes 
several folds, chiefly on the liver, before it reaches the anus. 
The salivary glands reach to the extremity of the gullet. 
The liver is divided into five lobes, which give rise to two 
ducts that open into the pylorus. 
The circulating system consists of two ven cavee, which 
give out numerous branches to the pulmonary cavity. The. 
aérated blood is conveyed by several ducts to a simple mem- 
 branaceous systemic auricle. Between the auricle and ven- 
tricle there are two valves. The ventricle is more w uscu- 
lar than the auricle. The arteries, which take their rise 
from a single aorta, are characterised by a peculiar opacity, 
and whiteness of colour, as if they were filled with milk. 
The organ of viscosity nearly encircles the pericardium. 
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