

SANGUIFEROUS SYSTEM. 455 



The venous blood of the system is returned to the great ven- 

 tral and branchial sinuses and transmitted through the gills, 

 and when arterialized is conveyed upwards to the dorsal part 

 of the trunk by the branchial veins, to be poured into the cavity 

 of the heart by two orifices on its upper surface (1 19. B.) The 

 venous and the arterial openings of the heart are provided 

 with valves ; this organ is already furnished, as in mollusca, 

 with a delicate pericardium which has often been mistaken 

 for an auricle, and there appear to be numerous venous sinu- 

 ses developed on the superficial parts of the trunk, besides 

 those connected with the branchial arteries. There is thus 

 only k single cavity of the heart developed, from the gradual 

 concentration of the extended dorsal vessel, in the highest of 

 the articulated tribes, which accords with the general muscu- 

 lar activity of this sub-kingdom, and this single ventricle is 

 systemic as in other invertebrata and in the embryos of the 

 vertebrated classes. And in tracing the successive phases 

 of the development of this muscular cavity in the highest 

 decapods, it is observed to pass through the adult conditions 

 presented by the inferior orders of Crustacea, and by all the 

 lower articulated classes. 



FOURTH SECTION. 



Sanguiferous System of the Cydo-gangliated or Molluscous 



Classes. 



The existence of a muscular ventricle on the great vascular 

 trunks of the system, by interrupting the equable current of 

 the blood, necessitates the development of an auricle to re- 

 lieve the veins from regurgitation and distention, and to per- 

 fect its own function ; and as these two cavities are not syn- 

 chronous in their development, we observe a higher grade of 

 the sanguiferous system in the existence of an auricle, super- 

 added to the ventricle, throughout all the classes of mollusca. 

 These two cavities are systemic, as the heart in other inverte- 

 brata, and this high condition of the vascular system accords 

 with the complicated structure of, the digestive, the glandular, 

 and other nutritive organs of these animals. As the respira- 

 tion of the mollusca is almost always aquatic and limited, 

 their muscular energy and movements are languid, and the 



