CIRCULATION IN THE CRUSTACEA. 



2.37 



shows the relative situation of these vessels when 



isolated, and viewed from 

 behind, in the 3Iaia 

 squmado. c, c, are the 

 venae cavae ; e, e, the 

 venous sinuses ; f, f, the 

 branchial arteries ; g, the 

 gills, or branchiae, and 

 I, I, the branchial veins, terminating in the heart, l.* 

 The heart of the lobster is situated immediately 

 under the carapace, or shell of the dorsal region of 

 the thorax, in a plane posterior to the stomach, 

 where it is not liable to be pressed against the re- 

 sisting shell, when the stomach is distended. 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 continual and rapid 

 renewal of nutrient fluid, and render necessary the 

 introduction of further refinements into its struc- 

 ture. The supply of blood to the heart, being in a 

 constant stream, produces a gradual dilatation of 

 the cavity which receives it ; and the muscular 

 fibres of that cavity are not excited to contraction, 

 until they are stretched beyond a certain point. 

 But in order effectually to drive the blood into 

 every part of the arterial system, where it has great 



* Such is the account given by Audouin and Milne Edwards, in 

 the Ann, Sc. Nat. xi. 283 and 352, It would appear, however, 

 that some of the branchial arteries are direct branches from the ar- 

 terial trunks which arise from the heart. See the Catalogue of the 

 Museum of the Royal College of Surgeons, ii. 137. 



