ORGANS OF CIRCULATION. 243 



Fishes is exhibited by the Amphioxus or Lancelot. The blood is 

 quite colorless, and has not as yet been found to contain any cor- 

 puscules. The hearts, however, are numerous. There is found, 

 1st. An Arterial Heart, a tube of uniform thickness placed below the 

 branchial thorax in the middle line where the branchial artery in 

 other Fishes is situated, but without any trace of a pericardium ; it 

 is continued for a short extent backward as far as the end of the 

 oesophagus, where it makes a curve and joins the tubular hearts of 

 the venae cavae. 3d. The Bulbilli of the branchial arteries, which, 

 given off regularly from the arterial heart, are continued into the 

 angles between each pair of branchial arches, and represent the 

 commencement of the branchial arteries ; in young individuals we 

 find twenty -five, but in older specimens fifty, such branchial hearts 

 on either side. The branchial veins probably bring the blood into 

 the aorta that lies beneath the vertebral column, but in addition 

 to these, the blood enters the aorta through, 3d. The Cardiform 

 aortic arches ; it is a double contractile ductus Botalli arising from 

 the median heart. 4th. A Portal heart, long and tubular in form 

 like a vessel, and contracted throughout its whole length, runs along 

 the ventral side of the intestine, and extends to the extremity of the 

 caecum. 5th. A heart of the vena cava, which lies on the opposite or 

 dorsal side of the intestine, and is also tubular in form. Both venous 

 hearts contract alternately. This structure of the vascular system 

 obviously reminds us of that in the Annelida, where numerous pulsa- 

 ting heart-like vessels also occur. 



The Cyclostomi are also characterized by many remarkable pecu- 

 liarities in their vascular system, but to describe them in the present 

 work would be entering into too great detail ; one distinguishing 

 feature, however, is the want of a muscular or contractile bulbus 

 arteriosus, the trunk of the branchial artery exhibiting a uniform 

 structure. In Lepidosiren annectens we find a bulbus arteriosus, and 

 a single auricle and ventricle. In L. paradoxa we meet with a right 

 and left auricle imperfectly separated, the former receiving the pul- 

 monary vein, the latter the venae cavae. 



Among the Osseous and Cartilaginous Fishes the number of hearts 

 is occasionally found to be also incred. Thus in Chimsera an 

 elongated fusiform accessory heart is always developed upon the two 

 axillary arteries destined to supply the pectoral fins. - Similar axillary 

 hearts also occur in Torpedo, butiiot in Raia. 



In the Eel there is found, upon both sides of the last caudal ver- 



