Embryology and Growth of Fishes 141 



ance, must also be established: the sense-organs — eye, ear, and 

 nose — mouth and anus, and one or more gill-clefts. 



Among the different groups of fishes the larval changes are 

 brought about in widely different ways. These larval pecu- 



FiG. 104. — Larva of Common Eel, Anguilla chrisypa (Rafinesque), called Lepto 

 cephotus grassii. (After Eigenmann.) 



liarities appear at first of far-reaching significance, but may 

 ultimately be attributed, the writer believes, to changed environ- 

 mental conditions, wherein one process may be lengthened, 

 another shortened. So, too, the changes from one stage to 

 another may occur \^'ith surprising abruptness. As a rule, it 

 may be said the larval stage is of longest duration in the Cyclo- 

 stomes, and thence diminished in length in sharks, lung-fishes. 

 Ganoids, and Teleosts; in the last-named group a very much 

 curtailed (i.e., precocious) larval life may often occur. 



The metamorphoses of the newly hatched Teleost must 

 finally be reviewed; they are certainly the most varied and 

 striking of all larval fishes, and, singularly enough, appear 

 to be crowded into the briefest space of time ; the young fish, 

 hatched often as early as on the fourth day, is then of the 



Fig. 10.5. — Larva of Sturgeon, Acipenser sturio (Linna?>is). (After Kupffer, 



per Dean.) 



most immature character; it is transparent, delicate, easily 

 injured, inactive ; within a month, however, it may have assumed 

 almost every detail of its mature form. A form hatching three 

 millimeters in length mav acquire the adult form before it be- 

 comes much longer than a centimeter." 



Peculiar Larval Forms. — The young fish usually differs from 

 the adult mainly in size and proportions. The head is larger 



