HEART OF FISHES. 473 



in Fishes than in higher Vertebrates. Its relative position to the 

 ventricle varies in different species, and permanently represents as 

 many similar variations displayed temporarily during the course of 

 the heart's developement in birds and mammals ; thus in the heart 

 of Scorp&na scrofa., as in the Myxinoids, the auricle is posterior to 

 and in the same longitudinal line with the ventricle : in the 

 Perch, fig. 311, c, Carp, Sole, and Eel, it has advanced to the 

 same transverse line, on the dorsal and left side of the ventricle : 

 in the Sturionidje and other Ganoids it extends more forward, 

 dorsad of both ventricle and bulbus arteriosus, and the heart, 

 including the venous sinus, is now bent into a sigmoid form. The 

 walls of the auricle are membranous, with thin muscular fasciculi 

 decussating and forming an open network ; but these are closer 

 and stronger in the Sun-fish, Sturgeons, and Plagiostonies. The 

 cavity is simple, but its inner surface is much fasciculated in the 



Sun-fish and Sturgeon, where the ends of the valves of the sinus 



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are attached to the strongest muscular bands. Only in the 

 Lepidosiren is there any vestige of a septum, and this is reticu- 

 late. The auricle communicates by a single orifice, commonly 

 with the dorsal or the anterior part of the ventricle : this is 

 guarded usually by two free semilunar valves ; but in the 

 Sturgeon, their margins and their surface next the ventricle are 



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attached to numerous ( chords tendinea?.' In the Orthagoriscus 

 the auricular aperture is guarded by four semilunar valves, the 

 two smaller ones being placed at right angles with and on the 

 auricular side of the two larger and normal valves : their margins 

 are free. 



The ventricle, fig. 311, A, usually presents the form of a four- 

 sided pyramid, one side dorsad toward the auricle ; one angle 

 ventrad, and the base forward. In the Lepidosteus and Poly- 

 pterus, however, it is pyriform : in the Pike it is lozenge-shaped : 

 in the Lophius, as in the Myxinoids and Lampreys, it is oval : in 

 most Plao;iostomes its transverse diameter is the longest, as if 



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preparatory to a division. Its cavity is, however, simple in all 

 fishes. The parietes of the ventricle are very muscular, and the 

 fibres are redder than those of any other part of the muscular 

 system ; but the colour is less deep in the ground-fishes than in 

 those that swim nearer the surface, and enjoy more active loco- 

 motion and respiration. The exterior muscular fibres decussate 

 and interlace together irregularly and inextricably ; but the 

 deeper-seated ones form more regular layers, the innermost being 

 transverse and circular, and separating readily by slight decom- 

 position from the outer and more longitudinal layers. Some of 



