CIRCULATION IN MOLLUSCA. 241 



corresponding to the two trunks of the branchial 

 veins. 



A great number of bivalve Mollusca exhibit the 

 singular peculiarity of the lower portion of the 

 intestinal tube passing through the cavity of the 

 heart : a structure probably designed to admit of 

 the direct passage of nutritious fluids, by transuda- 

 tion through the coats of the intestine, into the 

 general reservoir of the blood. 



The plan of the circulation is modified in a 

 remarkable manner in a small tribe of Mollusca, 

 which have been lately associated together under 

 the title of the Tunicata, and of whicli the Ascidia 

 may be taken as the type. In this animal, the 

 elongated form of the organ of impulsion might 

 lead us to regard it rather as an aorta than as a 

 heart. The compound ascidise, consisting of a 

 number of individuals connected together by pe- 

 duncles branching from a common polypiferous 

 stem, and possessing each its own heart, and systems" 

 of respiration and of nutrition, have yet a circula- 

 tion which is common to the whole group. The 

 creeping tube which unites the individuals com- 

 posing this animal republic, is the channel for two 

 separate currents of blood running in opposite 

 directions. One of these currents enters the as- 

 cidia by a branch through its peduncle, and pro- 

 ceeds directly to the heart, the contractions of 

 which propel one part of the blood into the gills, 

 and another part into the systemic system of vessels, 

 whence both portions being collected by veins, the 

 united returning current passes out of the animal 

 by its peduncle, forming the other stream of the 

 stem, and proceeds to circulate in the vessels of the 



VOL. II. R 



