=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. [Illustration: 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. [Illustration: FIG. 7.--Porcupine-fish, _Diodon hystrix_ (Linnæus). Tortugas Islands.] [Illustration: FIG. 8.--Thread-eel, _Nemichthys avocetta_ Jordan and Gilbert. Vancouver Island.] [Illustration: FIG. 9.--Sea-horse, _Hippocampus hudsonius_ Dekay. Virginia.] [Illustration: FIG. 10.--Harvest-fish, _Peprilus paru_ (Linnæus). Virginia.] [Illustration: FIG. 11.--Anko or Fishing-frog, _Lophius litulon_ (Jordan). Matsushima Bay, Japan. (The short line in all cases shows the degree of reduction; it represents an inch of the fish's length.)] The pine-cone fish (_Monocentris japonicus_) adds strong fin-spines to its bony box, and the porcupine fish (_Diodon hystrix_) is covered with long prickles which keep away all enemies. Among swift fishes, there are some in which the body is much deeper than long, as in _Antigonia_. Certain sluggish fishes seem to be all head and tail, looking as though the body by some accident had been omitted. These, like the headfish (_Mola mola_) are protected by a leathery skin. Other fishes, as the eels, are extremely long and slender, and some carry this elongation to great extremes. Usually the head is in a line with the axis of the body, but in some cases, as the sea-horse (_Hippocampus_), the head is placed at right angles to the axis, and the body itself is curved and cannot be straightened without injury. The type of the swiftest fish is seen among the mackerels and tunnies, where every outline is such that a racing yacht might copy it. The body or head of the fish is said to be compressed when it is flattened sidewise, depressed when it is flattened vertically. Thus the _Peprilus_ (Fig. 10) is said to be compressed, while the fishing-frog (_Lophius_) (Fig. 11) has a depressed body and head. Other terms as truncate (cut off short), attenuate (long-drawn out), robust, cuboid, filiform, and the like may be needed in descriptions.