SANGUTFEROUS SYSTEM. 457 



cular valves were shown by Chiaje to be already sufficiently de- 

 veloped in ascidice to check the flow of mercury when injected 

 in a direction contrary to that of the natural current of the 

 blood. The venous blood of the system and the nutriment 

 absorbed from the wide and highly vascular intestine appear 

 to be transmitted by venous trunks to the branchial arteries 

 arid the gills before they are conveyed to the heart ; and the 

 same plan of structure is more or less perceptible in the dif- 

 ferent forms of simple and compound tunicata ; as shown by 

 Savigny, Chiaje, Cuvier, Schalck, Meckel, and other anato- 

 mists, although their descriptions do not always coincide. 



The different parts of the heart and of the whole vascular 

 system are more distinct and more complicated in the conchi- 

 ferous animals, than in the tunicata, but they are constructed 

 according to the same type in these two classes of acephalous 

 mollusca, which consist entirely of aquatic and branchiated 

 animals. The heart is always systemic, and is placed as in 

 the former class in the dorsal part of the abdomen. It con- 

 sists of an auricle, commonly divided into two lateral parts, 

 which receives the aerated blood from the gills, and of a 

 ventricle, also sometimes divided into two lateral parts, which 

 transmits the blood to the aorta and systemic vessels. The 

 blood is colourless or of a bluish-white colour, and rarely of 

 a reddish hue. The venous blood is collected from the 

 system by venous trunks, and transmitted directly to the 

 branchial arteries to be spread over the large free pendent 

 reticulate and pectinated folds of the gills. From the lateral 

 position and the symmetrical development of the branchiae 

 and of the margins of the mantle, the palleal veins (Fig. 134. 

 a. a.) returning from the ciliated and aerated covering of the 

 thoracic cavity, and the branchial veins (134. b. b.) returning 

 the aerated blood from the gills, are separated to a distance 

 from each other on the two sides of the body. The two 

 sinuses which receive this aerated blood, and which consti- 

 tute the auricle (134. c. c.) of the heart, are therefore placed 

 apart from each other, and form two distinct cavities, in 

 almost all the conchifera. Their form is somewhat trian- 

 gular, with their largest side towards the branchial veins, 

 and their largest angle towards the ventricle, and their pa- 

 rietes, though thin, present distinct muscular fasciculi. They 

 have several openings for the branchial and the palleal veins, 



