SANGUIFEROUS SYSTEM. 473 



here extended above the ventricle, and it advances more for- 

 wards over the ventricle in the plagiostome fishes, as the rays 

 and sharks, than in the osseous and inferior fishes, where it 

 has undergone a less deviation from its primitive embryo 

 position. In higher classes the auricles advance to the ante- 

 rior part of the ventricles, and the whole are compactly 

 embraced in a small pericardial cavity. The cavity of the 

 pericardium, in the plagiostome fishes, communicates by dis- 

 tinct openings with the peritoneal cavity of the abdomen, and, 

 by means of the two lateral anal orifices, with the exterior 

 medium, as in the cephalopods ; and the heart and bulbus 

 arteriosus of the sturgeon are covered with a loose glandular 

 mass, like the glandular vesicles enveloping the venae cavse 

 of the cephalopodous mollusca. 



The auricle is more capacious and has thinner muscular 

 parietes than the ventricle, and it generally extends so much 

 transversely as to envelope the upper or dorsal side of the 

 ventricle, and to appear beyond it on each side, or to surround 

 it entirely during its own diastole. Although the margins 

 of the auricle are often angular, they do not develope the 

 fringed auricular extensions common in higher classes, and 

 the muscular bands form distinct fleshy columns in the inner 

 surface. The auriculo- ventricular orifice is situated near the 

 anterior and superior part of the ventricle, and is furnished 

 with two strong semilunar valves, with loose free margins, 

 like those of the auricular orifice of the sinus venosus. Some- 

 times these folds of the ventricular orifice, or mitral systemic 

 valve of the hot-blooded vertebrata, consist of three or four 

 parts, and in the cartilaginous fishes their margins are 

 strengthened by connecting tendinous cords. The ventricle 

 is comparatively small, but with thick muscular parietes 

 often separable into two laminae, and has generally an an- 

 gular and tapering or pyramidal form, in the osseous fishes, 

 being broad, oblique, and flat towards the tendinous dia- 

 phragm, and becoming smaller towards the anterior opening 

 into the bulbus arteriosus. In the higher cartilaginous 

 fishes the ventricle is more rounded, depressed and extended 

 transversely, like that of chelonia a preparation for the 

 longitudinal division of this cavity into two parts, which is 

 gradually effected in the class of reptiles. Its interior is 

 provided with thick, prominent, fleshy columns ; its colour 



