THE MARSIPOBRANCmi. 109 



ama. There are no jaws ; but the palatopterjgoid, the quad- 

 rate, the hyoraandibular, and the hyoidean apparatus of higher 

 Vertebrata, are imperfectly represented (Fig. 30, /*, g, h). In 

 some genera a basket-like cartilaginous apparatus strengthens 

 the walls of the oral cavity ; while, in others, such a framework 

 supports the gill-sacs. 



The Mars'ipohranohil possess neither the pectoral nor the 

 pelvic pair of limbs, nor their arches. Horny teeth may be 

 developed upon the roof of the palate, or upon the tongue, or 

 may be supported by peculiarly developed labial cartilages. 

 The alimentary canal is simple and straight, and the liver is 

 not sac-like, but resembles that organ in other Vertehrata. 



The heart has the usual piscine structure, consisting of a 

 single auricle preceded by a venous sinus, a single ventricle, 

 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 peritonaeum. 



In 3Iyxme the portal vein is rhythmically contractile. 



The cardiac aorta, which is continued from the bulb, dis- 

 tributes its branches to the respiratory organs. These consist 

 of antero-posteriorly flattened sacs, which communicate directly 

 or indirectly, on the inner side, Avith the pharynx, and, exter- 

 nally, with the surrounding medium. 



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

 open externally by as many distinct apertures. Internally, 

 they communicate with a long canal, which lies beneath the 

 oesophagus and is closed behind, while anteriorly it communi- 

 cates freely with the cavity of the mouth (Fig. 32, Pr). 



The kidneys are well developed, and have the ordinary ver- 

 tebrate 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 high- 

 er Vertebrata — 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 thalamencephalon; the hind- 

 brain, into metencephalon and myelencephalon (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 membranous tube, without further distinction 

 into canals and vestibule. 



The Marsipobranchii differ remarkably, not only from the 

 fishes which lie above them, but from all other vertebrate ani- 

 mals, in the characters of the olfactory organ, which consists of 



