246 CHITONID^E. 



scending 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 just 

 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 

 necessary, but I am anxious to show that this curious con- 

 trivance to effect a particular object has nothing in it essen- 

 tially 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 venae cavse to the great 

 arterial vein of the respiratory organ, from whence it is distri- 

 buted to its minor arteries, 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 advanced 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 Articu- 

 lata are comparatively obsolete, or mere continuous fluctuating 

 cylinders or inflations. A short O3sophagus conducts to the 

 stomach, which is an irregular subcylindrical cavity, about 

 double the diameter of the pyloric extremity ; it traverses the 

 body, forming a sudden curvature like the doubling of a horse- 

 shoe, and returns across the body with the posterior portion 

 parallel to the anterior one, commencing at the pyloric orifice 

 a very long intestine of five or six transverse or oblique folds, 

 supported by the liver, and disemboguing as the rectum, at 

 the centre of the posterior extremity between the bran- 

 chiae. This is quite different in the Articulata, in which the 

 intestine runs without inflexions through the middle of the 

 body. 



For further particulars we refer to the description of the 

 type, as well as for the liver, ovary, and foot, all of which pre- 

 sent no essential variation from the molluscan type, except the 

 double oviduct, if such be the case. 



The Chitons are best illustrated by the Patelloid section 



