254 LECTURE X. 



reaction of the oxygenous element; and for this, the most important 

 and efficient cause of its conversion into arterial blood, a contractile 

 cavity, with strong muscular walls, is provided, in order to impel the 

 blood to the organs especially destined to effect its decarbonisation 

 and oxygenation. The propelling organ is called the ' heart ' {Jig. 

 61. J9, q). the respiratory organs the ' gills ' or branchia3 {ih. t, ti), 

 since 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 predominant 

 muscular organ of circulation : a great vein answering to the ' vena 

 cardinalis ' extends forwards along the caudal region, beneath the 

 chorda dorsalis, above the kidney {fig. 46. h) ; and as it extends 

 along the branchial cesophageal 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 rythmically, and represents rudimentally the 

 branchial heart of the Myxinoids : the cardinal vein {ba) divides 

 anteriorly, and supplies the short vascular processes {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 ten- 

 tacula, (/,/), whence it returns to the pharynx by the labial vessels 

 which there unite together, and with the inferior trunk of the vas- 

 cular system, or arches, of the branchial pharynx. The free vas- 

 cular processes {gg) seem to me to perform most distinctly the func- 

 tion of gills, and they are so referred to in the characters of the 

 suborder Phcu-yiigo-branchu (p. 47.) : but they may be homologous 

 with the supralabial tentacles of the Ammocete. 



In the Myxinoids a heart consisting of an auricle and a ventricle 

 is situated, like the pulsating tube or sinus of tlie 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 

 ' columnar carnea3 ' or ' chordae tendineaj.' The artery, single here as 

 in all Fishes, rises from the fore-part of the ventricle with a pair of 



