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 {191}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. [Illustration: 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. [Illustration: FIG. 109.—A, Scale of _Balistes capriscus_, × 20; B, scale of _Monacanthus scopas_, × 20. (After Hertwig.)] The concentric rings observable (Fig. 105) on the surface of many Teleostean scales are an index to the age of the {192}Fish.[172] The formation of these rings depends on the fact that the lines of growth on the surface of the scale are more widely separated from one another on that portion of the scale formed during summer, and relatively closer together on that part which is formed during the winter; the more rapid growth in the warmer season probably being due to favourable conditions as to food and temperature, and the retarded growth of the colder season to the reverse. Hence, by counting the alternating zones of close-set winter lines and less closely approximated summer lines of growth, a reliable clue may be gained as to the age of the Fish. In the Dipnoi,[173] as in Teleosts, the scales are enclosed in dermal pockets, and exhibit a regular, imbricated disposition in oblique rows (Fig. 304, A). In shape they are nearly cycloid, or slightly oval, with the long axis coinciding with that of the body. Structurally, also, they bear some resemblance to Teleostean scales, although differing in details. On the outer surface of the scales there are numerous small conical spines. No significance, other than as an example of evolutionary convergence, can be attached to the resemblance between the scales of Fishes so widely separated as the Dipnoi and the Teleosts. [Illustration: FIG. 110.—_Hypostoma commersonii_. A scale from the periphery of the caudal fin, × 50; one of the spines (_s_) is implanted in its socket (_s′_). (From Hertwig.)]