CHAPTEE XIII. 



GROWTH AND VARIATION OF FISHES. 



Fig. 82.— Mouth of 

 Larva of Petromy- 

 zon branchialis. 



Changes of form normally accompanying growth (after ab- 

 sorption of the vitelline 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 in adult relatively to the size of the 

 head ; and again, the head is larger rela- 

 tively to that of the body. Changes amount- 

 ing to metamorphosis have been hitherto 

 observed in Petromyzon only. In the lar- 

 val condition (Ammoccetes) the head is 

 very small, and the tootliless 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; 

 the eyes grow ; and the dorsal 

 fin is divided into two divisions. 

 and Ana- 

 canths the embryonal fringe from 

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



Fig. 83. — Mouth of Petromyzon 



fluviatilis. 

 mx, Maxillary tooth ; md, Mandi- 



buiary tooth ; I, Lingual tooth ; In Malacopterygians 



s, Suctorial teeth. 



