LARVAL FISHES : THEIR METAMORPHOSES. 127 



Fishes of this kind, often for several months 

 after they leave the egg-shell, bear about with 

 them a very considerable amount of the remains 

 of that portion of the egg which constitutes what 

 is called the food yolk, in a flask-shaped bag with 

 a long neck, attached to the under surface of the 

 body (fig. 11). The mouth of this flask opens into 

 the intestine, and thence the contents of the bag 

 pass up into the gut 

 as required. At least 

 this is the state of 

 things at first ; later 

 the connection with 



the gut is CUt off, and FIG. ll. A larval dog-fish. 



the last remains of 

 the yolk are absorbed by the blood-vessels alone. 

 Why this curious method of absorbing the yolk 

 should be, depends upon the very extraordinary 

 fact that, the gullet or food-pipe, at first quite 

 tubular, later closes up and becomes quite solid, 

 so that all swallowing becomes absolutely im- 

 possible. During this period the advantage of 

 the pendant yolk-sack in open communication 

 with the gut is obvious. Its absorption later, 

 after the reopening of the proper food passage by 

 the blood-vessels, is as much a matter of con- 

 venience as for the sake of nourishment. The 

 explanation which has been given to account for 

 this curious closure of the gullet cannot be dis- 

 cussed here. In other larval fishes, such as luug- 

 fish, " ganoids," and the higher " teleostean " 

 forms, of which we may instance the salmon, 

 perch or cod-fish, the yolk-sack is relatively 

 smaller and packed away beneath the body, not 



