40 



Natural History of Molluscous Animals : — 



— ? 



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each side, of considerable size, 

 and direct their course along 

 the superior margins of the 

 branchial laminae, to which they 

 furnish numerous branches. 

 These form two series ; one for 

 the internal face of the external 

 leaf, and the other one for the 

 external face of the internal 

 leaf of the branchiae ; but in 

 consequence of the many ver- 

 tical and anastomotic branch- 

 lets, a close vascular network 

 is the result of the whole ar- 

 rangement. The veins, draw- 

 ing their supply from this net- 

 work, run backwards in a 

 direction parallel to the arte- 

 ries, and form a similar net- 

 work, but on the opposite faces 

 of the branchiae. 



As it offers some excep- 

 tions to this account of the 

 distribution of the blood-ves- 

 sels in the Conchifera in gene- 

 ral, I am tempted to extract 

 for you the interesting descrip- 

 tion of the circulation in Te- 

 redo navalis, as given by Sir 

 Everard Home : — " The 

 heart," he says, " is situated 

 upon the back of the animal, 

 near the head; consisting of 

 two auricles [Jig, 28.*), of a 



h 



u 



Wl 



* Figure of Tferfedo navalis, show- 

 ing the heart and other internal or- 

 gans, of the natural size, exposed in 

 a posterior view, a a. The boring- 

 shells, separated and turned back ; 1, 

 the digastric muscle ; c, the intestine 

 passing over it j dd^ the testicles ; e e, 

 the auricles of the heart ; //, the 

 ventricle ; g g, the artery going to the 



head; h h, the vessels from the branchiae going to the heart; ii, the 



branchiae or gills ; k k, ducts of the testicles, traced through their course ; 



//, a strong substance, with transverse fibres, having a pile upon it, to 



strengthen this, the weakest part of the animal. 



