212 



VERTEBRATE ANIMALS. 



The heart in fishes may be regarded as essentially a 

 branchial 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 (a) receives 

 the venous blood which 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" (m), which acts as a kind of 

 additional ventricle. By the ventri- 

 cle and bulbus arteriosus the venous 

 blood is driven to the gills, 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 

 FIG. 104. Diagram of the circuia- essential peculiarity of the circulation 

 fi* *!&&SS?1E; of fishes consists in this, that the ar- 

 venous system is left light. aAu- terialized blood returned from the 



ricle, receiving the venous blood . ,, .. .. , , 



from the body; Ventricle; m gills IS propelled through the gen- 



Bulbus arteriosus; n Branchial i Tr ac^l a rvf +1-,^ Korlv /cTrcf^rmV 



Korl 



artery, carrying the venous blood DOd 



to the gills (&); c Great systemic vessels) without being sent back to 



vessel, carrying the pure blood ,11, T j/i f i ^ i 



to the tissues. the heart. In the JLancelet, alone 



of all fishes, there is no single heart, 



and the circulation is effected by means of contractile dila- 

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

 fish (Lepidosiren) 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 Lancelet, are 

 most of them red. 



While the respiration of all fishes is truly aquatic, most 



