58 EVOLUTION, OH LE 
a sort of slime as a covering for its body, and remaining in 
this torpid condition, it breathes by means of lungs until 
the return of the water, when it rises to the surface and 
breathes by its gills. Hence the Lepidosiren is both Fish and 
Amphibian.. As regards its respiration, it is truly an Amphi- 
bian. It differs from the ordinary fish in the structure of 
its heart, which is composed of three chambers in the 
Lepidosiren and Amphibian (Siren, Frog, etc.), whereas in 
the Fishes the heart is composed of only two. The Lepi- 
dosiren and Polypterus both have the spiral valve in the 
intestine, so characteristic of the Sharks. The air-bladder 
in Polypterus, and the lungs of Lepidosiren, are the same 
in their structure as regards the arteries of these parts and 
the relations of their air-ducts. The form of the brain 
is the same in Lepidosiren and Polypterus. The skull 
of Lepidosiren is intermediate between the gristly and 
the bony fishes. The backbone is gristly; in this re- 
spect it agrees more with the Fishes than with the Am- 
phibia. In the structure of the liver apparatus and the 
limbs it agrees with the Amphibia. What is the Lepido- 
siren? Is it a Fish, or is it an Amphibian? The Lepido- 
siren is the intermediate form linking the Fishes and 
Amphibia together, and is more closely allied to the 
Ganoid Polypterus than any living fish. Among the fossil 
Ganoids the Coccosteus would represent the Lepidosiren 
should its skeleton be fossilized. The Ganoid fishes, 
although intermediate between the Sharks and common 
bony fishes or Teliosts, have many affinities with the 
Amphibia: thus, the Amia and Lepidosteus, among the 
Ganoids, have the air-bladder filled with air-vesicles and re- 
sembling strongly the lung of the Amphibia. So the back- 
bone of the Lepidosteus in the ball-and-socket joint of the 
pieces forming its spine differs from all Fishes, and agrees 
with many of the Amphibia. The structure of the Ganoids, 
Lepidosiren, and Amphibia seems to warrant the conclusion 
