SANGUIFEROUS SYSTEM. 471 



larity of the branchial hearts in that genus. In the loligopsis 

 the two aortal trunks come off from the anterior and the 

 posterior apex of the longitudinally extended ventricle, both 

 have a slight bulbous enlargement at their origin, and the 

 genital artery forms merely a branch originating, from the 

 commencement of the great dorsal or cephalic aorta. So 

 that in this highest of all the invertebrated classes, only two 

 of the great cavities of the heart, a ventricle and an auricle, 

 are yet distinctly developed ; and notwithstanding their 

 diversities of form and their occasional division, these two 

 cavities continue with great regularity throughout the mol- 

 luscous subkingdom. 



FIFTH SECTION. 



Sanguiferous System of the Spini- Cerebrated, or 

 Vertebrated Classes. 



There is great uniformity of plan in the different condi- 

 tions presented by the vascular system in the vertebrated 

 classes, and in the several phases of development through 

 which it passes before arriving at its perfect form in each of 

 the divisions of this subkingdom. The heart is always situ- 

 ated on the anterior or ventral aspect of the alimentary canal, 

 and its compartments are always contiguous and enclosed in 

 a distinct pericardium. In the species which breathe solely 

 by t branchiae, whether permanent or deciduous, the heart 

 consists of a single auricle and a single ventricle, which are 

 atf once, branchial and systemic, and the blood is entirely 

 propelled by it through these respiratory organs. But 

 in al^ the pulmonated tribes there is a distinct auricle 

 superadded, for the reception of the aerated blood from the 

 lungs ; and in the hot-blooded classes there is always a bilo- 

 cular heart for the pulmonic blood^ and a similar heart for 

 the systemic, which have no communication with each other, 

 though enclosed in the same pericardium, and formed by the 

 subdivision of the same original cavities. The heart in- 

 creases in bulk as we ascend in the vertebrata, being smallest 

 in the fishes, amphibia and reptiles, and largest in the hot- 

 blooded tribes ; and there is always a hepatic portal circula- 

 tion, which increases in extent as we advance from the lower 



