=The Moorish Idols: Zanclidæ.=—The family of _Zanclidæ_ includes a single species, the Moorish idol or kihi kihi, _Zanclus canescens_. In this family the scales are reduced to a fine shagreen, and in the adult two bony horns grow out over the eye. The dorsal spines are prolonged in filaments and the color is yellow crossed by bars of black. _Zanclus canescens_ is a very handsome fish with the general appearance and habit of a _Chætodon_, but the form is more exaggerated. It is found throughout Polynesia, from Japan to the off-shore islands of Mexico, and is generally common, though rarely entering rock pools. _Zanclus eocænus_ is recorded from the Italian Eocene. =The Tangs: Acanthuridæ.=—In the next family, _Acanthuridæ_, the surgeon-fishes or tangs, the scales remain small and shagreen-like, the body is more elongate, the gill-openings still more restricted, and the teeth are flattened and incisor-like. The pubic bone is more elongate, and in all the species some sort of armature is developed on the side of the tail. The spinous dorsal in all is less developed than the soft dorsal. The species abound in the warm seas, especially about the tide pools, and are used as food. They undergo considerable changes with age, the caudal armature being developed by degrees. Nearly all are dull brown in color, but in some a vivid ornamentation is added. Fossil forms are found from the Eocene and later. Most of these are referable to _Teuthis_ and _Acanthurus_. [Illustration: FIG. 343.—_Teuthis cœruleus_ (Bloch & Schneider), Blue Tang. Mexico. Family _Teuthididæ_. ] The principal genus is _Teuthis_, characterized by the presence on each side of the tail of a sharp, knife-like, movable spine with the point turned forwards and dropping into a sheath. This spine gives these fishes their name of surgeon-fish, doctor-fish, lancet-fish, tang, barbero, etc., and it forms a very effective weapon against fish or man who would seize one of these creatures by the tail. The species have the center of distribution in the East Indies and have not reached Europe. Three species are found in the West Indies. The blue tang (_Teuthis cœruleus_) is chiefly bright blue. The common tang, _Teuthis chirurgus_, is brown with bluish streaks, while a third species, _Teuthis bahianus_, has a forked caudal fin. Very close to this species is _Teuthis crestonis_, of the west coast of Mexico, and both are closely related to _Teuthis matoides_, found from India to Hawaii. [Illustration: FIG. 344.—Brown Tang, _Teuthis bahianus_ (Ranzani). Tortugas, Fla. ]