754 MOLLUSCA. 
central part or brain, situated above the intestinal canal; of ganglia for the 
different orders of sense, as well as for the locomotive apparatus; of a tew 
visceral ganglia, together with conducting filaments or nerves. The brain 
consists of two similar parts, more or less connected, and situated above the 
csophagus. The ganglion of the organ of sight communicates with the 
brain, as well as that of the organ of hearing, when it exists. Besides the 
more or less immediate connection which exists between the two parts of 
the brain, above the esophagus, there is another which passes under the 
cesophagus, thus forming a sort of ring. These visceral ganglia appear to 
be only two in number. The principal one is commonly placed near the 
stomach, and sends forth filaments to the intestinal canal, and others which 
communicate with the brain, by means of the esophageal ring. For the 
locomotive apparatus, and the organs of general sensation, there is but a 
single ganglion on each side, which communicates with the brain, by a 
chord. From this ganglion proceed the filaments that supply the musculo- 
cutaneous envelope, and especially those which are subservient to general 
locomotion, such as the foot of the Gasteropoda and Trachelipoda, the wings 
of the Pteropoda, &c. The circulation is complete in the mollusca. The 
heart is situated, in general, in the back, above the intestinal canal. It is 
not contained in a true pericardium, but in a muscular cell of the imperfect 
diaphragm, which separates the visceral cavity from that of the- bronchial. 
It consists of an auricle, sometimes double, anda ventricle. The auricle 
varies in its forms, but is commonly oval, with very thin walls; afew mus- 
cular cords, however, are observed to traverse itsinterior. It communicates 
with the ventricle by a sort of contraction, frequently of considerable length, 
as for example in the genus Loligo, and by means of a narrow orifice, com- 
monly transverse, situated between two folds-of the inner surface of the 
ventricle, but without valves. The ventricle iyin general much larger, and 
varies much in form and direction. Its walls are-always much thicker than 
those of the auricle, and the transverse, muscular fasciculi, of which it is 
formed, are very distinct. From the extremity of the heart issues the 
arterial system, commonly by a single trunk, but sometimes, also, by two. 
There are no valves placed at the commencement of this vessel. Their 
walls are thicker than those of the veins, and are possessed of great elasti- 
city. Their distribution varies in a considerable degree, although there are in 
general two trunks, an anterior and a posterior. The former furnishes 
oranches to the head and its different parts, to the esophagus, and even to 
ithe organs of generation ; while the other sends ramifications to the stomach, 
the rest of the intestines, the liver, and the secreting organs of generation. 
The veins have their walls extremely thin, and frequently so confounded 
with the tissue of the parts, as to be with difficulty distinguished. They 
constitute only two systems, one which comes from ail parts of the body, 
and the other from the respiratory organ, there being no system of the vena 
porte. The venous radicles of the general system of the body, after repeat- 
