THE LIFE-HISTORIES OF FISHES 163 



haddock, plaice, halibut, sole, flounder, and, indeed, 

 most of the fishes used as food by man. On the 

 other hand, some fishes produce eggs which are 

 slightly heavier than sea-water, and, when spawned, 

 these at once sink to the bottom. Such are 

 termed " demersal " eggs, and the best example of 

 this type is the egg of the herring (but not those 

 of the sprat, anchovy, or pilchard). There are 

 others, of course, but they are spawned by species 

 of no economic importance. These demersal 

 eggs nearly all have another character in common ; 

 that is, they are all furnished with an adhesive coat- 

 ing when spawned, and this causes them to stick 

 together. They are therefore always found in 

 masses adhering to pieces of sea-weed, stones, 

 shells, and other objects of the sea-bottom on 

 which they were deposited by the spawning fish. 1 

 Just as in the case of their breeding habits, so 

 the Elasmobranchii (gristly fishes) sharks, rays, 

 and dog-fishes differ very remarkably from the 

 Teleostei (or bony fishes) . The eggs of the former 

 fishes are large, and are always very few in number, 

 comparatively speaking. The empty capsules or 

 shells of the ray are familiar to anyone who has 

 grubbed among the debris lying on the sea-beach 

 along high-water mark, as " mermaids' purses." 

 They are oblong, black, leathery-looking objects, 



1 See Ehrenbaum, Wissenschaftliche Meeresuntersuchungen, Bd. vi., 

 Abth. Helgoland, Heft 2, 1904, for a series of most exquisite photo- 

 graphs of such egg masses in situ. 



