CHAPTER I. 



GENERAL REMARKS ON THE EGGS OF MARINE FISHES. 



Groups of Food-jis/ies. The British food-fishes fall natu- 

 rally under two groups (excluding the lamprey and the hagfish, 

 not treated of in this work), the first, that of the cartilaginous 

 fishes, the sharks, skates and dog-fishes, and the second, that 

 of the bony fishes, comprising all the remainder, and whose 

 life-histories form the special subject of this work. 



The former differ mostly from the latter in having a soft 

 cartilaginous skeleton throughout life, in having the gill-slits 

 separate from one another and not covered by a shield or 

 operculum, and in other points of anatomy, into which we 

 need not enter here ; but the great difference which concerns 

 us is that of development. The cartilaginous fishes have a 

 very few, very large eggs, often protected in a hard chitinous 

 skin which is popularly known as a " sea-purse " or " mermaid's 

 purse." This is usually laid by the parent either on sand or 

 in the vicinity of sea-weeds, amongst which the dark olive 

 colour of the " purse " serves to hide it, or again in other cases 

 the long filaments at the end are attached to various foreign 

 bodies. Most cartilaginous fishes carry the system of " pro- 

 tection " still further and the whole egg is carried about inside 

 the parent's body till the embryo is at a sufficiently advanced 

 stage to take care of itself, when it emerges from its parental 

 covering and commences its individual existence ; this is termed 

 a viviparous form of development. 



On turning to the bony fishes, such as the cod or the herring, 



