MOLLUSCA. 9 



the imperfect diaphragm which separates the visceral cavity from 

 that of the branchiae. It consists of an auricle, sometimes 

 double, and a ventricle. The auricle varies in form, but is 

 commonly oval, with very thin walls ; a few muscular cords, 

 however, are observed to traverse its interior. It communi- 

 cates 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, commonly transverse, situated be- 

 tween two folds of the inner surface of the ventricle, but with- 

 out valves. The ventricle is in general much larger, and varies 

 much in form and direction. Its walls are always much thick- 

 er than those of the auricle, and the transverse muscular fasci- 

 culi 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 elasticity. Their distribution varies in a considerable de- 

 gree, 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 all parts of the body, and the other from the respi- 

 ratory organ, there being no system of the vena portce. The 

 venous radicles of the general system of the body, after repeat- 

 edly uniting into trunks, arrive at the respiratory organ, where 

 they are converted into an arterial system, which ramifies through 

 its substance. 



From the capillary extremities of the branchial artery arises 

 the second venous system. The veins unite into branches, which 

 terminate in a large trunk, pouring its contents into the heart. 

 The colour of the blood is white or bluish. 



The organs of respiration vary considerably, not only with 

 respect to their form and the place which they occupy in the 

 animal, but also with respect to structure. In most of them 



