Fig. 107.—Centriscus scolopax. A, Scale from the orbital region, × 50; B, scale from the base of the pectoral fin, × 100. (From Hertwig.)
Amongst some of the usually scaleless Siluroid Fishes the scales assume a very peculiar structure. In Hypostoma[[171]] (Plecostomus) the sides and back of the Fish are covered by large bony plates, but on the under surface and on the fins these are replaced by much smaller ones. Both kinds, however, carry numerous small movable spines implanted in sockets (Fig. 110), a fact which suggests comparison with a stage in the development of the scales of Lepidosteus, when the independently formed and evanescent spines have not yet fused with the basal plates.
In other Teleosts, as in the Agonidae and some Triglidae, the body is completely cuirassed with large keeled bony plates. The singular appearance of many of the Plectognathi is largely the result of the curious modifications which their scales undergo. In some of the Coffer-Fishes (Ostracion) these structures assume the form of polygonal bony plates which suturally articulate with one another and enclose the trunk in a rigid cuirass, from which the scaleless tail protrudes behind (Fig. 438); while in some Globe-Fishes and Porcupine-Fishes (e.g. Tetrodon, Diodon) the prolongation of the scales into strong erectile spines equally well serves the purpose of protection (Fig. 439).
Most Teleostomi have the scales along the "lateral line" perforated by single or multiple apertures, through which the sensory canal communicates with the exterior.
Fig. 108.—A, Scale of Antennarius hispidus, × 100; B, scale of a young Malthe vespertilio, × 100. (After Hertwig.)
In a few Teleosts scales are entirely absent, as in most Siluridae; or they exist only as microscopic vestiges hidden in the skin, as in Eels; or, as in such naked forms as Antennarius marmoratus and Lepadogaster, and in some Siluridae, they become reduced to mere papillae of the dermis.
Fig. 109.—A, Scale of Balistes capriscus, × 20; B, scale of Monacanthus scopas, × 20. (After Hertwig.)