258 THE VITAL FUNCTIONS. 



system in the more highly organized tribes of 

 this class, we find the powers of the dorsal 

 vessel becoming more and more concentrated 

 in its anterior extremity ; till in the Decajjoda, 

 a family which comprehends the Lobster and 

 the Crab, we find this part dilated into an oval 

 or globular organ, with very muscular coats, 

 capable of vigorous contractions, propelling its 

 contents with considerable force into the vessels, 

 and therefore clearly entitled to the appellation 

 of heart. The distinction between arteries and 

 veins, which can scarcely be made with any 

 precision in the systems of the inferior tribes, is 

 here perfectly determined by the existence of 

 this central organ of propulsion : for the vessels 

 into which the blood is sent by its contractions, 

 and which, ramifying extensively, distribute it to 

 distant parts, are indisputably arteries; and con- 

 versely, the vessels, which collect the blood from 

 all these parts, and bring it back to the heart, 

 are as decidedly veins. The heart of the lobster 

 is situated immediately under the carapace, or 

 shell of the dorsal region of the thorax, di- 

 rectly over the stomach : its pulsations are very 

 distinct, and are performed with great regularity. 

 The importance of the heart, as the prime 

 agent in the circulation, increases as we advance 

 to the higher classes of animals, whose more 

 active and energetic functions require a con- 

 tinual and rapid renewal of nutrient fluid, and 

 render necessary the introduction of farther re- 



