542 
DR. A. GUNTHER’S DESCRIPTION OE CERATODUS. 
to Lepidosiren , of which now numerous examples have been kept in captivity, but none 
have shown a tendency to leave the water. I think it much more probable that this 
animal rises now and then to the surface of the water in order to fill its lung with air, 
and then descends again until the air is so much deoxygenized as to render a renewal of 
it necessary. The fish is said to make a grunting noise, which may be heard at night 
for some distance. This noise may be produced by the passage of the air through the 
oesophagus, when it is expelled for the purpose of renewal. From the perfect develop- 
ment of the gills we can hardly doubt that, when the fish is in water of normal compo- 
sition, and sufficiently pure to yield the necessary supply of oxygen, these organs are suffi- 
cient for the purpose of breathing, that the respiratory function rests with them alone, 
and that the lung receives arterial blood, returning venous blood, like all the other organs 
of the body. But when the fish is compelled to sojourn in thick muddy water charged 
with gases which are the product of decomposing organic matter (and this must be the 
case very frequently during the droughts which annually exhaust the creeks of tropical 
Australia), it commences to breathe air with its lung in the way indicated above. Under 
this condition the pulmonary vein carries purely arterial blood to the heart, where it is 
mixed with venous blood and distributed to the various organs of the body. If the 
medium in which the fish happens to be is perfectly unfit for breathing, the gills cease 
to have any function ; if only in a less degree, the gills may still continue to assist in 
respiration. Ceratodus, in fact, can breathe by either gills or lungs alone, or by both 
simultaneously. 
It is not probable that this fish lives freely out of the water, the limbs being much 
too flexible for supporting the heavy and unwieldy body, and too feeble generally to be 
of much use in locomotion on land. However, it is quite possible that it is occasionally 
compelled to leave the water, although I do not believe that it can exist without it in a 
lively condition for any length of time. 
Relative Situation of the Abdominal Viscera (Plates XXXIX. and XL.). 
The part which first attracts attention on opening the abdminal cavity is the extremely 
large and wide intestinal sac («), which fills the cavity nearly entirely. It is perfectly 
straight from its entrance below the diaphragm to the vent, without any circum- 
volutions, and of a nearly black colour, whilst the inside of the walls of the abdomen 
is of a silvery white. In the specimens examined it was throughout distended with 
food, apparently provided with thin walls, except below the stomachic region, where 
the walls were thicker, offering greater resistance to pressure. In the middle of its 
course it is crossed by six lines placed at regular intervals and indicating the insertion 
of the internal spiral valve. Below the last line the intestine is gradually contracted to 
its end. 
This large intestinal sac is fixed by a ligament (b) to the ventral surface of the cavity; 
this very peculiar ligament commences from the first turn of the spiral valve, and is con- 
tinued to the end of the intestine, fixing it, not exactly along the median line of the 
