THE MAESIPOBEANCHII. 123 



jes. The alimentary canal is simple and straight, and 

 the liver is not sac-like, but resembles that organ m other 

 Vertebrata. 



The heart has the visual piscine structure, consisting of 

 a single auricle preceded by a venous sinus, a single ven- 

 tricle, and an aortic bulb, all separated from one another by 

 valves. This heart is contained in a pericardium, the cavity 

 of which communicates with that of the peritoneum. 



In Myxine the portal vein is rhythmically contractile. 



The cardiac aorta, which is continued from the bulb, 

 distributes its branches to the respiratory organs. These 

 consist of antero-posteriorly flattened sacs, which commu- 

 nicate, directly or indirectly, on the inner side, with the 

 pharynx, and, externally, with the surrounding medium. 



In the Lamprey there are seven sacs, upon each side, 

 which open externally by as many distinct apertures. Inter- 

 nally, they commtinicate with a long canal, which lies beneath 

 the oesophagus and is closed behind, while anteriorly it com- 

 municates freely with the cavity of the mouth (Pig. 32, Pr). 



The kidneys are weU developed, and have the ordinary 

 vertebrate structure, while the ureters open behind the 

 rectum. 



The brain, though very small, is quite distinct from the 

 myelon, and presents all the great divisions found in the 

 higher Yertebrata — ^that is to say, a fore-brain, mid-brain, 

 and hind-brain. The fore-brain is further divided into 

 rhinencephala, solid prosencephalic lobes, and a thala- 

 mencephalon; the hind-brain, into metencephalon and mye- 

 lencephalon (Fig. 31). 



The auditory organ is simpler than in other fishes, possess- 

 ing only two semicircular canals and a sacculated vestibule 

 in the Lamprey. In Myxine the whole organ is represented 

 by a single circular membranovis tiibe, without further dis- 

 tinction into canals and vestibule. 



The Marsipobrancliii differ remarkably, not only from the 

 fishes which lie above them, but from all other vertebrate 

 animals, in the characters of the olfactory organ, which 

 consists of a sac placed in the middle line of the head, and 



