[CHAPTER II]
THE EXTERIOR OF THE FISH
Form of Body.—With a glance at the fish as a living organism and some knowledge of those structures which are to be readily seen without dissection, we are prepared to examine its anatomy in detail, and to note some of the variations which may be seen in different parts of the great group.
In general fishes are boat-shaped, adapted for swift progress through the water. They are longer than broad or deep and the greatest width is in front of the middle, leaving the compressed paddle-like tail as the chief organ of locomotion.
Fig. 6.—Pine-cone Fish, Monocentris japonicus (Houttuyn). Waka, Japan.
But to all these statements there are numerous exceptions. Some fishes depend for protection, not on swiftness, but on the thorny skin or a bony coat of mail. Some of these are almost globular in form, and their outline bears no resemblance to that of a boat. The trunkfish (Ostracion) in a hard bony box has no need of rapid progress.
Fig. 7.—Porcupine-fish, Diodon hystrix (Linnæus). Tortugas Islands.