Dr. Johnston on Scottish Mollusca. 45 



a determinate direction propel the water over it in corre- 

 sponding currents, and thus supply a never-failing stream of 

 unbreathed water*. 



The varied position of the branchiae necessarily implies 

 some corresponding peculiarities in the arrangement of the 

 circulating system. It is a general rule in physiology that the 

 heart shall be placed in the immediate vicinity of the organs 

 which air the blood ; and accordingly it is found situated, in 

 the Doris, far backwards, just anterior to the branchial circle ; 

 while in the other genera, which have the branchiae distributed 

 along the sides, its position is more forwards and nearly cen- 

 tral. It is a strong muscular organ, consisting of an auricular 

 and a ventricular cavity, separated by two semilunar valves, 

 and its inner surface is netted with numerous fleshy columns, 

 like the heart of a vertebrate animal. Receiving the blood, or 

 what, because of its cold and colourless qualities, Linnaeus 

 called the sanies, from vessels bringing it in a purified condi- 

 tion from the gills, it propels the stream forward into an aorta, 

 by whose divergent ramifications this is led to every part and 

 organ of the body. By venous vessels, nearly parallel but run- 

 ning in the contrary course, the blood is again returned, the 

 (—vessels as they trend inwards uniting repeatedly, until, after 

 many unions and coalescences, they form two or one large 

 trunks that serve the place of a pulmonic heart. In the Trito- 

 niada these vessels run parallel, two along each side of the 

 body, in correspondency with the position of the branchiae, and 

 give off a small branch to every tuft and filament of them ; but 

 in Doris they keep a more central and medial course. The 

 blood collected in them is effete and unfit to continue fife ; but 

 by numerous branchlets which depart from these trunks and 

 ramify through the gills, it is again dipersed and exposed to 

 the oxygenating medium before it re-enters the heart to begin 

 again the ceaseless circuit. 



The nervous system consists of four ganglions placed over 

 the origin of the oesophagus, from which filaments are sent 

 to the different organs, to the foot, and cutaneous envelope. 

 There is neither other ganglion nor plexus. From this sim- 



* Sharpey in Cyclop. Anat. and Phys. i. 620. 



