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 cartilagino'us 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 caecum 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 venae cavse, which 

 give out numerous branches to the pulmonary cavity. The 

 aerated 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 rruscu- 

 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. 



