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 protopterus 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 appendages. On these are distributed the blood-vessels, which constitute 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, Triglœ, 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 frœnum, or fold, forming a sort of rudimentary septum, rises from the floor of the ventricle; this frœnum 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 œsophagus; finally, 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,
