322 A HISTORY OF FISHES 



sea-floor. No evident connection seems to exist between the 

 habits of a particular species and the nature of its eggs, for 

 whereas the pelagic, plankton-feeding Herring {Clupea harengus) 

 produces demersal eggs that are deposited in adhesive masses 

 among gravel and shingle on the sea-bottom, the closely related 

 Sprat (Clupea sprattus), with exactly the same habitat, as well 

 as the Pilchard [Sardina) and the Anchovy (Engraulis), have 

 typical pelagic eggs which float separately near the surface. 

 The Angler (Lophius), a typical ground-dwelling form, has 

 pelagic eggs, but the Wolf-fish {Anarrhichas) , also living on or 

 close to the bottom, has large demersal eggs. 



Pelagic eggs are very much smaller than those of the demersal 

 type, and it is their small size and glassy transparency when in 

 the sea that helps to render them inconspicuous to other fishes. 

 Those of the Plaice {Pleuronectes) , which are to be regarded as 

 giants of their kind, never exceed two miUimetres in diameter. 

 Eggs of this type when developing in the ovaries or roes are 

 pinkish, opaque objects, but as they mature the granules of 

 yolk disappear and both they and their capsules become quite 

 translucent. A conspicuous feature of many pelagic eggs is the 

 presence of a single large oil-globule, forming a glistening 

 object moving about freely on the surface of the yolk. In 

 others the yolk itself may be partially or completely broken 

 up into small masses, giving the egg a characteristic appearance. 

 For the most part, they are non-adhesive, floating freely and 

 separately at or near the surface of the sea, but those of the 

 Angler {Lophius) are invested by a gelatinous outer coat and 

 unite together to form a transparent mass, which may be as 

 much as one hundred feet square in area. The little fish known 

 as Fierasfer also has adhesive floating eggs, forming a mass of 

 cylindrical shape, two or three inches in length. 



Of the fishes with demersal eggs, those breeding in fresh water 

 have, as a general rule, larger eggs than those spawning in the 

 sea. Among the largest are those of Gymnarchus of Africa, which 

 measure about ten millimetres in diameter, and those of some 

 of the Sea Cat-fishes (Ariidae), occasionally exceeding fifteen 

 millimetres. Among marine fishes, those of the Lump-sucker 

 (Qyclopterus) are about two and a half millimetres in diameter, 

 and of the Wolf-fish {Anarrhichas) about six milUmetres. Demer- 

 sal eggs may also be quite free and separate, as in the Salmon 

 (Salmo), Shad (Alosa), and other anadromous fishes, in which 

 case they are usually provided with fairly tough and smooth 

 outer membranes: more usually, however, they ha\e adhesive 



