224 VERTEBRATE ANIMALS. 



fin behind, forming a continuous fringe round the compressed 

 tail, and supported by cartilaginous rays. 



P 

 FIG. 110. Lepid o#iren annectens, the Mud-fish, p Pectoral limbs ; v Ventral limbs. 



The skull is composed of distinct bones, and there is a 

 lower jaw, but the notochord is persistent, and there are no 

 bodies of vertebrae developed. The respiratory organs are 

 twofold, consisting, firstly, of free filamentous branchiae or 

 gills, contained in a branchial chamber, which opens externally 

 by a single vertical gill-slit ; and, secondly, of true lungs, in 

 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 two auricles, divided from one another by an 

 incomplete partition. Lastly, the nasal sacs open behind into 

 the throat, and do not form closed chambers opening only by 

 the nostrils, as they do in all other fishes, except the Myxi- 

 noids. The two best-known species are the Lepidosiren 

 paradoxa from the Amazons, and the L. annectens from the 

 Gambia. They both inhabit marshy tracts, and both appear 

 to be able in the dry season to bury themselves in the mud, 

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

 till the rains of the wet season set them free 



