342 FISHES 



the external gills of the young tadpole, projecting freely from 

 the gill-arches, and in Protopterus vestiges of these gills are re- 

 tained throughout life; in Lepidosiren they disappear during 

 the metamorphosis. The larvae of these two genera have also, 

 like the young tadpole, a glandular organ of adhesion behind 

 the mouth. The eggs of the surviving fringe-finned Ganoid 

 Polypterus have not been discovered ; like Protopterus it 

 breeds in the rainy season, and Budgett obtained a single larva 

 from the river Gambia. This larva was about an inch long 

 with fin-rays already developed, and had on each side a long 

 feather-like external gill on the hyoid arch. 



The eggs of the sturgeons are extremely small in comparison 

 with the size of the adult fish ; in the common sturgeon the 

 diameter is only y 7 in. One of the most interesting facts 

 about the development of the sturgeon is that the larva is pro- 

 vided with teeth, although the adult is toothless. 



In the larvae of the American bony-pike {Lepidosteus) and 

 bow-fin {Amid) there are no external gills, such structures being 

 evidently not required in fishes hatched in a more temperate 

 climate and more aerated water than that of the African tropics, 

 where the more primitive forms above mentioned breed. In 

 both Amia and Lepidosteus the eggs are adhesive, and the 

 larvae are also provided with an adhesive organ, in this case situ- 

 ated not behind the mouth as in the Dipnoi, but at the end of 

 the snout. 



Among the Teleostei three kinds of eggs can be distin- 

 guished with respect to the properties of the vitelline membrane 

 and the conditions under which they undergo development ; 

 first, eggs of which the egg-membrane is hard and smooth and 

 the eggs themselves sink to the bottom like those of the Sal- 

 monidae and shads, second, those in which the membrane is 

 adhesive as in the herring and lump-sucker and most littoral 

 fishes, third, those in which the membrane is non-adhesive and 

 the whole egg is very transparent and floats in the sea, as in the 

 great majority of marine fishes. The majority of fresh-water 

 fishes produce eggs which are attached to fixed objects by the 

 surface of the enclosing membrane ; this is the case with the 

 perch, whose spawn is attached to water-weeds in comparatively 

 still water. In some cases the membrane is provided with long 

 filaments arising from it in two groups at opposite poles as in the 



