CHAP. XIIL] 



CIRCULATION. 



197 



In the other vertebrates there is a heart, but we see it perfecting 



c 



*~l U_ ^i ' I? ^ * ^> SsVO //' / *> 



^-i r-*-<: *- ^l r? NML-- v 



FIG. 18. 



Schematic figures showing the comparison of the modifications of the circulatory centres 

 in the mollusks : A, part of the dorsal trunk and of the transversal vessels of a worm ; 

 B, heart and auricle of a nautilus ; C, heart and ventricle of a lammelibranchian or loliginate ; 

 D, the same organ in an octopus ; E, heart and auricle of a gasteropod ; v, ventricle ; a, 

 auricle ; a c, cephalic artery ; a i, abdominal artery. The arrows indicate the direction 

 of the sanguineous current. 



itself little by little. The simplest form of the heart in verte- 



FIG. 19. 



Schema of the framework of 

 the great trunks, of the dif- 

 ferentiated apparatus of the 

 branchial vessels ; a, arterial 

 bulb ; 1 5, arterial arcs ; 

 o a", aorta ; c, carotid artery. 



FIG. 20. 



Heart, branchial arteries, and opercu- 

 lary branchia of the Lepidosteus 

 osseus; y, ventricle ; A A, auricle; 

 B, arterial muscular bulb ; a, trunk 

 of the branchial arteries ; 1, accessory 

 branchia (operculary) ; p, pseudo- 

 "branchia (vent branchia) ; 2, 3, 4, 5, 

 branchiae of the arcs. The arrows 

 indicate the direction of the blood. 



L 



brates exists in fishes. There, the cardiacal dilatation is merely 



