FISHES. R27, 
position of these pairs of fins admits of some variation in the 
different species; sometimes the pectoral fins are just above the 
ventral, and sometimes the latter approach the tail. The other 
fins are usually single. The cazda/ fin is the tail; the azaZ, which 
is sometimes double, is on the under surface, near the tail; while 
from the ridge of the back rise the dorsal fins, of which there are 
sometimes one, two, or three. Hence the maximum number of 
fins is ten—four in pairs, and six single. The fins are traversed 
by rays, which vary in their consistence in different species. 
It is needless to say that fish are perfect swimmers, being able 
to turn in any way with the greatest rapidity, to dart forward, or 
to stop suddenly at will, and with the greatest ease and precision, 
The “gait” of their motion differs just as the walk of the members 
of our species ; this is mainly owing to the different positions and 
size of the fins as well as the shape of the body. They do not 
all swim with the same strength. Some can resist the rush of the 
most violent waves; while others, like the tunny and the diodon, 
are borne upon the breast of. the currents, and transported, even 
against their will, from one place to another. 
A great variation is found in the form of the tail. Indeed, 
from the mode in which this caudal appendage is formed, the fish 
tribe is divided into two great classes. When the backbone is 
prolonged into the tail, one lobe of the caudal fin is much larger 
than the other, and therefore such fish are said to be heterocercal 
(unequal lobed). This construction will be noticed in any of the 
illustrations given of sharks. Upon this type were formed all the 
Paleozoic fish, which are now found fossilised in the geological 
formations below the new red sandstone. It was when the hetero- 
cercals were predominant, in the old red sandstone period, that 
fish-life on our globe reached its maximum. Ever since that now 
remote age, the fish have gradually retreated from the supremacy 
they held. 
The heterocercals are now the exceptions, most of the members 
of the finny race being homocercal, that is, having tails with equal 
lobes; the fact being that the tail is a veritable fin, appended to 
the end of the vertebral column; the backbone does not enter 
the tail. 
There is great diversity in tails. The Sea-horse (Hippocampus) 
