The concentric rings observable (Fig. 105) on the surface of many Teleostean scales are an index to the age of the 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.
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.)
All known fossil Dipnoi had scales of a similar character, although differing greatly in size in different genera. In some (e.g. Dipterus) a layer of enamel-like substance invests the exposed portions of the scales.
CHAPTER VIII
THE SKELETON
All Fishes possess an internal skeleton which, in order that it may be distinguished from the more superficial scaly exoskeleton described in Chapter VII., is termed the endoskeleton. The latter consists (i.) of an axial part, including the vertebral column and the skull; and (ii.) of an appendicular portion, consisting of the skeleton of the limbs and their supporting pectoral and pelvic girdles.
The Vertebral Column.[[174]]—The individual segments or vertebrae which, arranged in a linear series, collectively form the vertebral column, are highly complex structures, each being formed by a number of vertebral elements, the sum total of which constitutes a vertebra. Perhaps the best conception of the nature of vertebral elements is to be gleaned from the study of such primitive Fishes as the Elasmobranchs, in which not only are all the vertebral components present, but they are less modified by suppression and fusion than in most other Fishes, and on this account they afford a convenient introduction to the study of the puzzling eccentricities of vertebral structure in other groups. Selecting any common Dog-Fish, such as Scyllium canicula, and starting with an early embryonic stage, it may be stated that the first indication of a vertebral column is the formation of the notochord, which, invested by its chordal sheath, extends from the tip of the tail to a point on the under surface of the brain just behind the hypophysis or pituitary body.
