316 FISHES AND FISHING 



of shock or excitement. Such changes are, perhaps, 

 best observed in captive examples, and may often be 

 noticed if a fish is kept in a bucket on deck for a 

 short time after capture. 



Eggs. 



Sharks and rays and allied fishes are either vivi- 

 parous or else lay large eggs enclosed in horny capsules 

 or " purses " (Fig. 205) ; the dissection of pregnant 

 females will often serve, in viviparous species, to show 

 how many young are born in one litter ; or, in ovip- 

 arous species, to connect an egg capsule with the 

 parent fish. 



Teleostean fishes, although sometimes viviparous, 

 usually produce a comparatively large number of 

 small eggs, examples of which are shown in Fig. 207. 



These eggs may be either " pelagic " — that is, 

 buoyant and floating in the water at whatever depth — 

 or " demersal " — that is, attached to some object, or 

 lying among gravel or debris at the bottom. 



The pelagic eggs of different species of fish vary from 

 about o«5 to 2 millimetres in diameter, while demersal 

 eggs are commonly somewhat larger, and are seldom less 

 than about 1 -5 millimetres in diameter. Their diameter 

 is most easily measured by putting them under a micro- 

 scope of low power, with an eyepiece ruled in squares 

 of about 0-05 millimetre value with the objective 

 employed. 



It should be remembered that the eggs of teleostean 

 fishes are very readily fertilized artificially, if ripe ex- 

 amples of both sexes are forthcoming for the purpose. 

 To effect artificial fertilization, all that is necessary is 

 to stroke gently the belly of the fish from in front 

 backwards until eggs or milt are extended, and to 



