548 THE POPULAR SCIENCE MONTHLY. 



" Thus the cocoon is a protecting sheath formed of the mucous secretion. 

 The abundant secretion of mucus, in the first place, coats and strengthens the 

 walls of the burrow made by the protopterus, and hence the subterraneous 

 canal which it had excavated had its sides smooth, and as it were polished. 

 Then, after the animal has reached the required depth, the secretion becomes 

 still more abundant, and the mucus dries, forming a membranaceous envelope 

 of remarkable structure." 



The animal doubles itself up in its envelope, the tail being brought 

 up in front of the head ; the mouth is free, and through it passes the 

 air needed for respiration, which, of course, is exclusively pulmonary, 

 owing to the conditions in which the animal lives. In fact, the pro- 

 topterus is able to respire in two ways, viz., either directly in the 

 atmosphere, or indirectly by separating dissolved oxygen from water 

 by means of its gills. 



The external openings to the latter are two small apertures, one 

 on each side of the neck. Each of these gives access to a chamber 

 of moderate capacity, in which are floating certain filamentous ap- 

 pendages. On these are distributed the blood-vessels, which consti- 

 tute but ill-developed tufts. In water, the animal respires by means 

 of these ; when it lives in its burrow, it respires by means of its lung. 



Most fishes have, beneath the vertebral column, a sort of capsule, 

 which seems to act the part of a floating apparatus. By means of 

 this, the fish can rise in the water or descend at pleasure; it is known 

 as the air-bladder. The sounds emitted by certain fishes, Triglce, for 

 example, are caused by vibrations communicated to the gases in this 



organ. 



In the protopterus the air-bladder discharges the physiological 

 functions of a lung when the animal can no longer respire through 

 the gills. To attain this end, it divides up into a number of little 

 cellular lobes, over the walls of which are spread a multitude of 

 blood-vessels, containing blood to be oxygenated, though it is only 

 partially venous. To prevent mixture of the two kinds of blood, 

 that which has respired the oxygen and that which has discharged 

 its physiological function, the auricle becomes divided in two by a 

 partition. The left chamber receives the red blood, just as in the 

 higher animals. A muscular frce?ium, or fold, forming a sort of rudi- 

 mentary septum, rises from the floor of the ventricle ; this frcenum 

 acts as a piston, preventing the return of the blood into the vessels 

 by contracting when the heart contracts. 



The air enters either through the mouth or through the nostrils, 

 which debouch near the posterior margin of the upper lip ; thence it 

 passes into a trachea, which traverses the wall of the oesophagus; 

 filially, having entered a sort of membranous sac, through two large 

 openings, it reaches the lungs, whereof there are two, and which are 

 like the lungs of serpents. 



These singular animals, being, as we have said, truly amphibious, 



