i64 OUTLINES OF ICHTHYOLOGY. 



by two males, one on each side, so as to secure the 

 impregnation of the greatest amount of spawn, 

 the range of milt being immensely increased by 

 diffusion in water. 



After this process, the eggs are left amongst the 

 gravel, or sticking to weeds or other substances, the 

 glutinous nature of each egg supplying the means 

 of adhesion ; and when the time (which constantly 

 varies) for the hatching of the ^<g<g arrives, the 

 young fish breaks the shell, which has become 

 very thin, and escapes. Artificial production — by 

 impregnating the eggs taken from the female fish, 

 with the milt taken from the male — is now exten- 

 sively practised. 



There are a few exceptions to the rule of fishes 

 being oviparous. Amongst Sharks and Rays one 

 or two species are viviparous, that is, producing 

 their young alive and complete in all respects. In 

 these fishes, which belong to the cartilaginous 

 series, the organs of reproduction are more compli- 

 cated in their structure than those of bony fish, 

 resembhng, in fact, the sexual organs in reptiles. 



A few other species, again, such as the Dogfish, 

 and some of the Rays or Skates, bring forth their 

 young enclosed in bony cases. Their empty shells, 

 known by the names of " mermaids' purses," 

 "skate-barrows," &c., are frequently picked up on 

 the sea-shore, and will be familiar to most of us, 

 though very possibly we may not have been 

 acquainted with their origin. 



