90 A HISTORY OF FISHES 



increase in the speed might be expected to lead to a change in 

 the shape of the areas into which the skin is wrinkled. A study 

 of extinct fishes provides a tolerably clear story, first of the 

 gradual development and perfection of heavy armour, followed 

 later by its gradual but no less definite decline when greater 

 freedom of movement became all - important. Interesting 

 transitional forms have been discovered, of which Aetheolepis 

 from the Jurassic of Australia has retained the articulated 

 ganoid plates on its relatively immobile trunk, whereas they 

 have been transformed into thin, overlapping scales on the 

 flexible tail. In other fossil forms all the scales are of the 

 ganoid type, but whereas those on the body are articulated 

 with one another, those in the tail region are quite free. 



The typical scaly covering in a modern Bony Fish may best 

 be studied in such fishes as the Carp [Cyprinus) or the Perch 

 (Perca). In the Carp (Fig. 43) the whole body with the ex- 

 ception of the head and fins is protected by a number of regu- 

 larly arranged, thin, flexible, bony plates known as cycloid 

 scales, overlapping one another like the tiles on the roof of a 

 house. Each scale, which is shaped roughly like the human 

 finger-nail, has the front end inserted deep into a pouch in the 

 dermal layer of the skin, the hinder portion being quite free. 

 The overlapping (imbrication) of the scales is, from a mechani- 

 cal point of view, important, and may be explained in the 

 following manner. The muscles attached to the dermis tend 

 to exert a somewhat unequal pull, and, therefore, to depress 

 the scale areas, particularly at their front margins; in this way 

 the growing scale is forced to lie obliquely, and at a later stage 

 its hinder end appears through the skin (Fig. 37B). Only this 

 free portion is covered by an epidermal membrane. In the 

 Perch the scales are very much smaller, but have exactly the 

 same arrangement. A closer study of a scale under the hand 

 lens shows that its hinder end is provided with a row of small 

 tooth-like spines instead of being smooth as in the Carp (Fig. 

 39e). Such a scale is known as ctenoid (comb-like). 



The arrangement of scales just described may be taken as 

 typical of most Bony Fishes, but a large number of deviations 

 from this occur, either in the direction of degeneration or of 

 further specialisation. Cycloid (smooth) and ctenoid (spinate) 

 scales are not so widely different as would appear from the 

 above descriptions, as the one type is Hnked up with the other 

 by an almost complete series of intermediate stages. For 

 example, the posterior edge of a cycloid scale may be wavy 



