194 THE VITAL FUNCTIONS. 



form large receptacles, which are called sinuses, where the 

 blood is allowed to accumulate, and where, by the muscularity 

 of the expanded coats of the vessels, it receives an additional 

 force of propulsion. From the branchias the blood is re- 

 turned by another set of veins to the elongated heart for- 

 merly described, and propelled by that organ into the sys- 

 temic arteries. Fig. 354 shows the relative situation of 

 these vessels, when isolated and viewed from behind in 

 the Muja squinado. c, c, are the venae cavse; e, e, the ve- 

 nous sinuses above-mentioned; f, f, are the branchial ar- 

 teries; G, the gills, or branchiae; and i, i, the branchial 

 veins terminating in the heart l.* 



In the Mollusca, the heart acquires greater size, compared 

 with the other organs, and exerts a proportionally greater 

 influence as the prime mover in the circulation. In the de- 

 velopment of its structure, in the different orders of this class, 

 a beautiful gradation may be perceived: the Branchiopoda 

 having two hearts, one placed upon each of the two lateral 

 trunks of the branchial veins; the Gasteropoda having a 

 single heart, furnished with an auricle; and the Jicephala 

 being provided with a heart, which has a single ventricle, 

 but two auricles, corresponding to the two trunks of the 

 branchial veins.t 



The most remarkable variety of structure is that exhibited 

 by the Cephalopoda. We have already seen, in the Crusta- 

 cea, dilatations of the venae cavse, at the origin of the branch- 

 ial arteries: but in the Nautilus the dilatations of the branch- 

 ial veins are of such a size, as to be almost entitled to the 

 appellation of auricles. The Sepia, in whose highly organ- 

 ized system there is required great additional power to pro- 

 pel the blood with sufficient force through the gills, is pro- 

 vided with a large and complicated branchial apparatus; and 



• A minute account of the organs of circulation in the Crustacea is g-iven 

 by Audouin and Milne Edwards, in the Annates des Sciences Naturelles, 

 xi. 283 and 352, from which work the above figure is taken. 



f A great number of bivalve Mollusca exhibit the singular peculiarity of 

 the lower portion of the intestinal tube traversing through the cavity of the 

 heart. 



