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 

 oesophagus. 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 oesophagus, there is another which passes under the 

 oesophagus, 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 oesophageal 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 Ptcropoda, &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, and a ventricle. The auricle 

 varies in its forms, but is commonly oval, with very thin walls ; a few mus- 

 cular cords, however, are observed to traverse its interior. 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 is in 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 

 branches to the head and its different parts, to the oesophagus, and even to 

 the 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 bodv, 

 and the other from the respiratory organ, there being no system of the vena 

 porta. The venous radicles of the general system of the body, affp>- repeat- 



