190 MARINE AND FRESHWATER FISHES 



black upon a tawny ground, after the manner of a leopard, 

 adapt themselves readily to the artificial conditions of a 

 marine aquarium, and breed freely in the tanks. Contrary 

 to the preceding forms which are all viviparous, these 

 Ground Sharks deposit eggs, usually two at a time, enclosed 

 in horny cases several inches long, not unlike those of the 

 Skates, but having their extremities produced into long 

 cord-like tendrils which during deposition are wound tightly 

 round stones, sea-weeds, and other submarine objects, the 

 eggs being thus securely anchored until the escape of the 

 young fish. The gradual development of the embryo 

 Dog-fish, which in its earlier days possesses tufted external 



FIG. 27. SPOTTED DOG-FISH (Scy Ilium stellaris}. 



gills, like a Tadpole, may be distinctly observed through the 

 more transparent egg-cases, and affords one of the most 

 interesting and instructive exhibitions furnished by a well- 

 ordered aquarium. The Spotted Dog-fish are essentially 

 nocturnal in their habits, rarely active, unless when fed or dis- 

 turbed, during broad daylight, but waking into life with the 

 approach of dusk, and then swimming swiftly to and fro or 

 around their tanks with a peculiarly graceful gliding motion. 

 The eye-coverings in these fish are remarkably complex ; 

 within the first or outer eyelid, which closes upwards 

 like that of a bird, is a second protective envelope, acting 

 as a diaphragm, and which throughout the day is, with the 



