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 

 cephalus 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 with 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. 105. Larva of Sturgeon, Acipenser sturio (Linnaeus). (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 may 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 



