384 
POPULAR SCIENCE REVIEW. 
Amphibian, but, as we have seen, there are numerous gradations, 
leading by various transitions from a single cavity up to a 
highly complex cellular organ, which both in structure and 
function is indisputably a lung. The fishes which most closely 
resemble the amphibians belong to the orders Ganoidei and 
Protopteri. Now, in the first of these orders we find a swim- 
bladder, often cellular, as well as an air-duct. The curious bony 
pikes of S. America (Lepidosteus), remarkable for the hard 
bone-like scales with which they are covered, and the allied genus 
Polypterus of some of the African rivers, belong to this order. 
Lepidosteus has an undivided air-bladder with a wide pneumatic 
duct ; Pol3^pterus possesses a double air-bladder and a pneumatic 
duct communicating with the oesophagus. If we turn to the 
Protopteri, we shall find in the Lepidosiren the transitions 
completed, for here we find a double lung-like air-bladder with 
air-duct, glottis, and pulmonary vein — and this respiratory 
apparatus, be it remembered, is at certain periods functionally 
identical with the lungs of air-breathing vertebrates; for the 
Lepidosiren, after the rains have ceased to flood the river 
Gambia, finds -itself left behind in the mud of the retreating 
waters ; the scorching rays of a tropical sun compel the fish to 
burrow into the mud, in which it forms a kind of cocoon of 
hard-baked clay. How is the fish to live in this changed 
locality ? As an inhabitant of the water the respiration was 
effected by means of its gills alone, as in ordinary fishes ; but 
how is the circulation to be maintained now it is a terrestrial 
animal ? Professor Owen tells us in distinct terms : — What- 
ever amount of respiration was requisite to maintain life during 
the dry months is effected in the pulmonary air-bladders ; its 
short and wide duct or trachea, the oesophageal orifice of which 
is kept open by a laryngeal cartilage, introduces the air directly 
into the bladders ; the blood transmitted through the bran- 
chial arches to the pulmonary arteries is distributed by their 
ramifications over the cellular surface of the air-bladders, 
and is returned arterialised by the pulmonary veins. A 
mixed venous and arterial blood is thence distributed to the 
.system and again to the air-bladders. When the Lepidosiren 
resumes its true position as a fish, the branchial circulation 
is vigorously resumed,” &c. In the Lepidosiren, then, we 
have an instance of an animal which is a fish at certain 
periods of its existence and an amphibian at others ; and, as it 
seems to me, we have in the Lepidosiren a striking illustration 
of the transition of an organ, originally constructed for one 
purpose, into one for a wholly different purpose. As Mr. 
Darwin has said, All physiologists admit that the swim- 
bladder is homologous, or ideally similar in position and 
structure, with the lungs of the higher vertebrate animals ; 
