436 Comparative Morphology of Chordates 



In size, the teleosts range from the 10 mm. Philippine Pandaka to 

 such giants as the halibut, the swordfish, and tuna, which attain lengths 

 of 12 to 15 feet, and the grotesque marine sunfish (Mola) which may 

 be 8 or 10 feet long and weigh about 1 ton — in spite of the fact that it 

 appears to be only half of a fish (Fig. 337:9). Otherwise known as the 

 "headfish," its trunk has been foreshortened to such an extent that 

 the animal seems to consist mainly of head. 



So far as is known, no teleost has ever attained the size of the 

 larger sharks. The shark is a strong swimmer and, considering its bulk, 

 can make good speed. But the teleost, relative to size, can do very much 

 better. Compared to the shark, a teleost has bone in place of cartilage, 

 a somewhat more highly differentiated muscular system, improvement 

 in the "streamlining" of the external form of the body, the great 

 advantage of the hydrostatic air-bladder, and, over all, a more highly 

 elaborated nervous mechanism. These differences combine to make 

 possible such speed and precision of control as to enable the teleost to 

 arrive at the perfection of aquatic locomotion. A shark at its best is a 

 clumsy swimmer compared to a salmon swimming upward through a 

 waterfall. 



The Chondrostei, Polypterini, and Holostei are, in many re- 

 spects, anatomically intermediate between Elasmobranchii and Tele- 

 ostei. The relatively abundant fossilized remains of fishes from the 

 late Paleozoic and Mesozoic geologic ages indicate that fishes more or 

 less closely resembling the members of these three now living groups 

 were the most numerous and widely distributed fishes of those times. 

 The name "Ganoidei" is sometimes used in a general and not very 

 sharply defined way to indicate all of these fishes, extinct and living, 

 which, both in geologic time and in anatomic characteristics, are inter- 

 mediate between "cartilaginous fishes" (Elasmobranchii) and "bony 

 fishes" (Teleostei). The words "Ganoidei" and "ganoin" are derived 

 from a Greek word meaning "brightness" and refer to the smooth and 

 shining outer surface of such ganoin-covered scales as those of Polyp- 

 terus and the gar pike. But not all so-called "ganoids" have ganoid 

 scales. 



Although anatomically intermediate between sharks and teleosts, 

 the phylogenetic position of the modern "ganoids," especially the 

 Chondrostei with their largely cartilaginous skeletons, is not at all 

 certain. Are they relatively primitive piscine relics which have per- 

 sisted down through the ages with little or no evolutionary progress, at 

 least in the matter of ossification? Or is it possible that, descended from 

 ancestors with highly ossified skeletons, they have undergone, to vary- 

 ing extent, progressive reduction in degree of ossification? In our 



