68 



A HISTORY OF FISHES 



fish {Apogon) (Fig. 28g). The evolution of spinous rays added 

 a new function to the fins, that of attack and defence, and 

 pugnacious fishes Hke the httle Sticklebacks (Gasterosteus) know 

 how to use their formidable dorsal and pelvic spines to the best 

 advantage (Fig. 28f). The spines vary greatly in different 

 fishes, both in height and thickness, and may even be soft and 

 flexible as in some of the Gobies and Blennies (Fig. 34f). The 



Flat-fishes (Heterosomata) 

 provide an example of 

 the secondary trans- 

 formation of spines into 

 soft rays. It is known 

 that these fishes have 

 evolved from spiny- 

 rayed forms not unlike 

 the Sea Perches, but 

 have become very much 

 modified for a life on 

 the sea bottom. In a 

 primitive form from 

 tropical seas {Psettodes) 

 the dorsal fin commences 

 well behind the head, 

 and the front part is 

 still supported by stiff 

 spines, but in all other 

 Flat-fishes the fin has 

 grown forward on to the 

 head, and the spines 

 have been reconverted 

 into flexible articulated 

 rays, thus allowing of 

 the wave-like movements 

 essential for progression 

 {cf. p. 27). In some 

 of the more specialised Soles (Synaptura), and in the Tongue- 

 soles (Cynoglossus) , both dorsal and anal fins are united with 

 the much reduced caudal, so that the three, together with 

 the pelvic, form a complete fringe round the body. 



In many of the Sea Perches and their alHes the spines are 

 unequal in size and strength, a somewhat delicate spine alter- 

 nating with a stout one. In the Pine-cone Fish {Monocentris) of 

 Japan the spines are particularly formidable and are curiously 



Fig. 29. LOCKING MECHANISM OF FIN-SPINES. 



A. Shoulder girdle and spines of pectoral fins 

 of a South American Cat-fish {Doras sp.), 

 ventral view, X f ; b. Dorsal fin-spines and 

 associated bones of the Trigger-fish (Balistes 

 sp.), lateral view, X f , 



