CHAPTER XIII. 



GROWTH AKD TAEIATION OF FISHES. 



Changes of form normally accompanying growth (after ab- 

 sorption of the viteUiae sac) are observed in all fishes ; 

 but in the majority they affect only the proportional size 

 of the various parts of the body. In 

 young fishes the eyes are constantly larger 

 than ia adult relatively to the size of the 

 head ; and again, the head is larger rela- 

 " tively to that of the body. Changes amount- 

 F' 82— Mouth of ^^S *° metamorphosis have been hitherto 

 Larva of Petromy- observed in Petromyzon only. In the lar- 

 zontonchiaus. ^^^ condition {Ammocoetes) the head is 

 very small, and the toothless buccal cavity is surrounded 



by a semicircular upper lip. The 

 eyes are extremely small, hid- 

 den in a shallow groove ; and 

 the vertical fins form a con- 

 tinuous fringe. In the course of 

 three or four years the teeth are 

 developed, and the mouth changes 

 into a perfect suctorial organ; 



Fig. 83.— Mouth of Petromyzon the cyes grow ; and the dorsal 



fluviatilis. . ,..,,. ■,■ ■ ■ 



ma;, Maxillary tooth; mi, Mandi- fi^ IS divided mto twO dlVlSlonS. 



huiary tooth ; i, Lingual tooth; jn Malacopterygians and Ana- 

 s. Suctorial teeth. , i j i i i ,. . c 



canths the embryonal irmge from 



which the vertical fins are developed, is much longer per- 



