470 ANATOMY OF VERTEBRATES. 



83. Heart of Fishes.- -The propelling organ is called the 

 ' heart,' fig. 308, n ; the respiratory organs the ( gills ' or branchiae, 

 ib. B, b ; fig. 312, i, 6 ; fig. 323, 2, 3, 4, 5 ; they submit the blood 

 to the influence of the air through the medium of the water in 

 which it is suspended or dissolved. 



There is only one known fish, viz. the Lancelet, in which a 

 venous or branchial heart is not developed as a compact and pre- 

 dominant muscular organ of circulation : a great vein answering 

 to the ' vena cardinalis ' extends forward along the caudal region, 

 beneath the chorda dorsalis, above the kidney, fig. 169, h ; and as 

 it extends along the branchial ossophageal sac gives vessels to or 

 receives them from the ciliated vertical bands or divisions of that 

 sac, which vessels communicate with a vascular trunk along the 

 inferior part of that sac. This trunk at its posterior end dilates 

 into a small sinus, ov, which pulsates rhythmically, and represents 

 rudimentally the branchial heart of the Myxinoids : the cardinal 

 vein, ba, divides anteriorly, and supplies the short vascular pro- 

 cesses, gg, which project above the pharyngeal orifice, ph, into the 

 wide buccal cavity : the blood oxygenized in these processes is 

 transmitted to the cerebral portion of the neural axis, to the 

 organs of sense, especially the sensitive integument of the head, 

 and to the jointed labial tentacula, f, f, whence it returns to the 

 pharynx by the labial vessels which there unite together, and with 

 the inferior trunk of the vascular system, or arches, of the branchial 

 pharynx. 



In the Myxinoids a heart consisting of an auricle and a ven- 

 tricle is situated, like the pulsating tube or sinus of the Lancelet, 

 far back from the head, in the beginning of the abdomen, where 

 it is inclosed by a fold or duplicature of the peritoneum, extending 

 between the cardiac end of the oesophagus above, and the anterior 

 liver below, and forming the homologue of the pericardium, which 

 sac communicates freely by a wide opening with the common 

 peritoneal cavity. The auricle is much longer than the ventricle : 

 it receives the blood from the common sinus by an orifice defended 

 by a double valve. The auricle communicates with the left side 

 of the rounded ventricle, the { ostium venosum ' having also a double 

 valve. There are no e columnar earner ' or ( chordae tendinese.' The 

 artery, single here as in all Fishes, rises from the fore-part of the 

 ventricle with a pair of semilunar valves at the ' ostium arteriosum ' 

 behind its origin, beyond which it slightly dilates, but has no 

 muscular parietes constituting a ' bulbus arteriosus.' In a large 

 Myxinoid (Bdellostoma cirratum, Dum.) the vessel from the heart 

 divides at once into two branchial trunks, reminding one of the 



