The special importance of a study of the anatomy of fishes lies in the
fact that fishes are on the whole undoubtedly the most archaic of
existing craniates, and it is therefore to them especially that we must
look for evidence as to the evolutionary history of morphological
features occurring in the higher groups of vertebrates.
In making a general survey of the morphology of fishes it is essential
to take into consideration the structure of the young developing
individual (embryology) as well as that of the adult (comparative
anatomy in the narrow sense). Palaeontology is practically dumb
excepting as regards external form and skeletal features, and even of
these our knowledge must for long be in a hopelessly imperfect state.
While it is of the utmost importance to pay due attention to
embryological data it is equally important to consider them critically
and in conjunction with broad morphological considerations. Taken by
themselves they are apt to be extremely misleading.
_External Features._--The external features of a typical fish are
intimately associated with its mode of life. Its shape is more or less
that of a spindle; its surface is covered with a highly glandular
epidermis, which is constantly producing lubricating mucus through the
agency of which skin-friction is reduced to an extraordinary degree; and
finally it possesses a set of remarkable propelling organs or fins.
The exact shape varies greatly from the typical spindle shape with
variations in the mode of life; e.g. bottom-living fishes may be much
flattened from above downwards as in the rays, or from side to side in
the Pleuronectids such as flounder, plaice or sole, or the shape may
be much elongated as in the eels.
_Head, Trunk and Tail._--In the body of the fish we may recognize the
three main sub-divisions of the body--head, trunk and tail--as in the
higher vertebrates, but there is no definite narrowing of the anterior
region to form a neck such as occurs in the higher groups, though a
suspicion of such a narrowing occurs in the young _Lepidosiren_.
The tail, or postanal region, is probably a secondary development--a
prolongation of the hinder end of the body for motor purposes. This is
indicated by the fact that it frequently develops late in ontogeny.
The vertebrate, in correlation perhaps with its extreme cephalization,
develops from before backwards (except the alimentary canal, which
develops more _en bloc_), there remaining at the hind end for a
prolonged period a mass of undifferentiated embryonic tissue from the
anterior side of which the definitive tissues are constantly being
developed. After development has reached the level of the anus it
still continues backwards and the tail region is formed, showing a
continuation of the same tissues as in front, notochord, nerve cord,
gut, myotomes. Of these the (postanal) gut soon undergoes atrophy.
_Fins._--The fins are extensions of the body surface which serve for
propulsion. To give the necessary rigidity they are provided with
special skeletal elements, while to give mobility they are provided with
special muscles. These muscles, like the other voluntary muscles of the
body, are derived from the primitive myotomes and are therefore
segmental in origin. The fins are divisible into two main
categories--the median or unpaired fins and the paired fins.
[Illustration: FIG. 1.--Heterocercal Tail of _Acipenser_. a, Modified
median scales ("fulcra"); b, bony plates.]
[Illustration: From _Cambridge Natural History_, vol. vii., "Fishes,
&c.," by permission of Messrs. Macmillan & Co., Ltd.