=The Fish's Nest.=--The sunfish in the spawning time will build some sort of a nest of stones on the bottom of the eddy, and then, when the eggs are laid, the male with flashing eye and fins all spread will defend the place with a good deal of spirit. All this we call instinct. He fights as well the first time as the last. The pressure of the eggs suggests nest-building to the female. The presence of the eggs tells the male to defend them. But the facts of the nest-building and nest protection are not very well understood, and any boy who can watch them and describe them truly will be able to add something to science.
=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.