248 VERTEBIiATE ANIMALS. 



two pairs of limbs, but these are in the form of awl-shaped organs, 

 each supported by a single, jointed cartilaginous rod. The pectoral 

 limbs have a membranous fringe inferiorlj', and the ventrals are 

 placed very far back. There is also a median hn behind, forming 

 a continuous fringe round the compressed tail, and supported by 

 cai'tilaginous rays. 



The skull is composed of distinct bones, and there is a lower jaw ; 

 but the notcjchord is jjersistent, and theie are no bodies of vertebrie 

 developed. The respiratory organs are twofold, consisting, firstli/, 

 of free filamentous branchias or gills, contained in a branchial 

 chamber, which opens externally by a single vertic:d gill-slit ; and, 

 secondly of true lungs, ia the form of a double cellular air-bladder 

 communicating with the gullet by means of an air-duct or windpipe. 

 Sometimes, if not always, there are rudimentary external gills as 

 well, placed on the side of the neck. The heart consists of a ven- 

 tricle, and of tiro aui'icle.s, divided from one another by an ineom- 

 ]ilete partition. Lastly, the nasal sacs oiien behind into the throat, 

 and (1(1 n(it form closed chambers opening only by the nastrils, as 

 they do in all ((ther fishes except the Myxinoids. The two best- 

 known species are tlie Lepidosiren paradoxa from the Amazon, and 

 the L. {Protopterus) aniiectens from the Gambia ; but the former is 



Fig. 179. — Ceratodus Forsttri, one of tli(3 Australian Mu(3-fishe.s, reduced in size. 



very rare and little known. They both inhabit marehy tracts, and 

 both appear to be able in the dry season to bury themselves in the 

 mud, and to furm a kind of chamber, in which they remain dormant 

 till the rains of the wet season set them free. 



In the fresh waters of Queensland, Australia, are found two other 

 remarkable MiK.l-fishes, belonging to the genus Ceraiodus. The 

 singular Cenitodus Forxteri (lig. 179), often called the '" Barra- 

 munda," grows to the length of six feet, and has the body covered 

 with Very Large cychjid scales. The fins are not awl-like, but con- 

 sist of a fringe of fin-rays surrounding a central lobe. The Barra- 

 munda, .seems to feed upo]i leaves, which it masticates by means 

 of cinious undulatecl bony ](la,tes or "molare," of which each jaw 

 cai'i'ies two. It ajjjiears occasionally to come to land, at which 

 times it doubtless lireathes liy means of its lungs ; but it has well- 

 developed gills also, and is es.sentially an a(piatic animal. 



