212 
THE LIVING WORLD. 
crawling, or jumping. The distinction in these respects between reptiles and 
fishes is not well defined unless the general characteristics be considered, in 
which structure is almost as important as organization or habits. The parti¬ 
cular line of demarcation, however, is found in the fact that fishes breathe 
water, so to speak, while reptiles breathe air direct. To be more explicit, a 
more perfect apparatus for aquatic respiration is seen in fishes; the gills are 
comb-like fringes supported on three or more bony or cartilaginous arches, and 
are composed of myriads of microscopic capillaries, by which the venous blood 
as it flows through them is exposed, in a state of minute sub-division, to 
streams of water. The gills are always covered, and the water which is taken 
in at the mouth escapes by the gill openings at ihe sides, hence the process is 
equivalent to breathing water, by which the blood is aerated, or arterialized. 
While fishes have lungs, they are imperfectly developed, being no more 
than the air-bladder, which is wholly rudimentary In the proteus and siren, 
however, which are described in this work, though under the division of 
reptiles , both gills and 
lungs are present, 
which serve to make 
them a connecting link 
■between fishes and 
reptiles, as difficult of 
classification as is the 
bat. Lungs are rel¬ 
atively largest in rep¬ 
tiles, but the air cells 
are few and large, and 
the blood capillaries 
are exposed to the air 
on only one side. In 
other words, fishes in¬ 
spire only, while rep¬ 
tiles, like birds and 
mammals, breathe 
through the mouth 
and nose. Turtles and tortoises, whose ribs are united together so as to con¬ 
stitute an inflexible shield, are compelled to swallow the air, whereas in other 
vertebrates air is drawn into the lungs by expansion and contraction of the ribs. 
Another distinction between fishes and reptiles is found in the sense of 
touch, which in the former resides in the lips, while in the latter it is most 
positive in the tongue. 
Other points of distinction exist between the orders, but except for the 
student of comparative zoology they are not interesting, and require too much 
classical research to warrant their description here. We will, therefore, proceed 
with a more general consideration of the order and species which properly 
belong under the head of cheIonia, a Greek word meaning tortoise. 
TURTLES OF A PRE-ADAMITE FERIOD. 
Ill the introductory remarks concerning the appearance and development 
of reptilian life, we saw how huge were the monsters that ploughed the prirn- 
EXTINCT PROTOSTEGA RESTORED. 
