456 SANGUIFEROUS SYSTEM. 



systemic position of their heart is thus necessary to accelerate 

 the circulation and nutriment of their inert frame. The 

 auricle is often divided, and sometimes also the ventricle, in 

 the mollusca, as we find the kidney, the spleen, the liver, 

 and many other organs, divided into parts in their lowest or 

 their earliest forms, but the functions of the parts are always 

 the same, and this divided condition of the cavities is often 

 necessary from the lateral position or the distance of the 

 branchiae, or to suit the general form of the body. Although 

 abounding in fibrin and globules, and affording a white coa- 

 gulum after death, the blood of the living mollusca is still 

 thin, transparent, and colourless, or of a pale bluish- white hue, 

 rarely red coloured, but the parietes of the vessels which 

 convey it are now always distinct and their three tunics are 

 commonly perceptible, as in the vertebrated classes. The 

 diversities presented by the circulating organs are greater 

 than in the articulated or the vertebrated tribes, and depend 

 chiefly on the remarkable differences of form found in mol- 

 luscous animals, on the difference of position and character 

 of their respiratory organs, and on the various grades of deve- 

 lopment presented by their general organization. 



The heart of the cynthia, and many other isolated forms of 

 tunicated animals is situated, as in the conchifera, in the ab- 

 dominal cavity, between the alimentary canal and the muscu- 

 lar enveloping mantle ; it is contained in a distinct pericar- 

 dium, and consists of a dilated muscular portion of the 

 great systemic artery, which was observed by Dicquemare 

 to pulsate in the living animal. This muscular pulsating and 

 transparent heart, consists of an elongated fusiform ventri- 

 cle, with a minute auricular dilatation observed by Chiaje to 

 be partially divided, as in the inhabitants of bivalve shells, 

 and to possess distinct valves at their venous orifices \ and 

 these cordiform cavities are so situate as to receive the arte- 

 rialised blood from the reticulate laminae of the gills, by two 

 branchial veins, into the bifid auricle, and thence into the 

 ventricle to be propelled through the single systemic aorta. 

 The auriculo-ventricular orifice is already provided with dis- 

 tinct valves, and the systemic aorta extends along the dorsal 

 part of the abdominal cavity towards the anus, between the 

 alimentary canal and the mantle, and following along the 

 course of the intestine. Both the auricular and the ventri- 



