212 



TERTEBEATE ANIMALS. 



The heart in fishes may be regarded as essentially a 

 hrancliial or respiratory heart, being concerned chiefly ■with 

 driving the venous and impure blood to the gills. It con- 

 sists in almost all cases of two 

 cavities, an auricle and a ventricle 

 (Fig. 104). The auricle («) receives 

 the venous blood vrhich has passed 

 through all the various parts of the 

 body, and propels it into the ven- 

 tricle (v). From the ventricle pro- 

 ceeds a single great vessel (the 

 "branchial artery"), the base of 

 which is usually developed into a 

 muscular cavity, the " bulbus arte- 

 riosus" (jTi), which acts as a kind of 

 additional ventricle. By the ventri- 

 cle and bulbus arteriosus the venous 

 blood is driven to the gUls, where it 

 is subjected to the action of the wa- 

 ter. The aerated blood is not re- 

 turned to the heart, but is driven 

 from the gills through all parts of 

 the body, the propulsive force neces- 

 sary for this being derived partly 

 from the heart, and partly from the 

 contractions of the muscles between 

 which the blood-vessels pass. The 

 Fie. 104.— Diagram of the Circuk- essential peculiarity of the circulation 

 te:'fs\IlreBentol"Te' of fishes consists in this, that the ar- 

 ■penous system is left light, a An- terialized blood returned from the 



ncle, receivTiig the Tenous blood ... , iit.i iji 



from the body; i> Ventricle; m gllls IS propelled through the gCn- 



Syrc^^?g?h^.:n?rbtd eral vessels of the body (systemic 

 tothe gills (»); c Great systemic vessels) without beinff Sent back to 



vessel, carrying the pm-e blood .i n'. xji x ix i 



to the tissues. the heart, in the JLiancelet, alone 



of all fishes, there is no single heart, 

 and the circulation is eflFected by means of contractile dila- 

 tations situated upon several of the vessels. In the Mud- 

 fish (JLepidosiren) the heart consists of two auricles and a 

 ventricle. In all cases the blood is cold, or, in other words, 

 has a temperature very little, or not at all, higher than that 

 of the surrounding medium. The blood-corpuscles (Fig. 99, 

 e) are always nucleated, and, except in the Lancolet, are 

 most of them red. 



While the respiration of all fishes is truly aquatic, most 



