Mr. W. Clark on the Chitonidse. 279 



the body, whilst the minor and posterior ventricle, after receiving 

 from the arcuated ducts of the auricles its quota of blood, serves 

 the remaining area ; but I could not detect a descending arterial 

 vein, which however must exist to convey the blood ; unless one 

 of the arteries of the ascending aorta doubles back and supplies 

 the part. But this conjectm-e is scarcely tenable, as we must then 

 suppose, that the blood conveyed by the arcuation of the auri- 

 cles to the lower inflation is thrown back again on them and the 

 larger ventricle, thus producing a useless periodic action and 

 counter-action. 



We will now endeavour to explain the probable causes and effects 

 of this unusual construction. It has been shown that the principal 

 ventricle of the heart is the propelling power of the blood to the 

 major part of the body, and the smaller one by its separate auri- 

 cular contact supplies the remainder ; we also have observed on 

 the connection of both parts of the heart by the strangulated and 

 without doubt valvular intervening portion, so that no blood can 

 pass between the two ; and it is clear that none is intended to 

 pass, as the auricles by distinct ducts supply each with its pro- 

 portion of the vital fluid ; consequently the mitral valves of the 

 ventricles are closed at their axes abutting on the strangulation, 

 at each simultaneous dilatation, and thus this constriction acts 

 as a, point d'appui and departure, and enables them to exercise a 

 full power of propulsion. 



We shall now perceive the reason for these separate inflations. 

 If one only had existed, with an ascending and descending aortic 

 vein of similar calibre, as much blood would be sent to a very 

 small area as to one seven times more extensive, and so great an 

 inconvenience would have resulted, that nature has created this 

 peculiar mode to effect a jnst distribution. I hope I have almost 

 proved that this anomalous structure is a consequence of the 

 posterior position of the heart. 



I have extended these remarks somewhat beyond what is ne- 

 cessary, but I am anxious to show that this curious contrivance 

 to effect a particular object has nothing in it essentially contrary 

 to the molluscan type ; in other respects, as in it, the circulation 

 is aortic, venous, and particular, the blood being brought from 

 the body by the vense cavse to the gi-eat arterial vein of the re- 

 spiratory organ, from whence it is distributed to its minor arte- 

 ries, and after aeration reverts by the branchial vein to the 

 auricles and heart, to repeat, as long as life lasts, the same course ; 

 consequently it is completely molluscan, and appears more ad- 

 vanced in composition than that of the Annelida, by the presence 

 of a much more effective motive power of the heart and auricles, 

 which in the Articulata are comparatively obsolete, or mere con- 

 tinuous fluctuating cylinders or inflations. A short oesophagus 



