_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. FIG. 2.--_Cladoselache._ (After Dean.)]