412 



FISHES 



large oil globule on the surface of the yolk, and not infrequently 

 the yolk becomes partially or completely broken up into small 

 masses. Pelagic eggs are always non-adhesive and free, and they 

 invariably belong to marine Pishes. Amongst the British food 

 Fishes which produce pelagic ova may be mentioned the Gadidae 

 {e.g. Cod, Whiting, Hake, Ling), the Pleuronectidae {e.g. Turbot, 

 Brill, Sole, Plaice), Scombridae {e.g. Mackerel), Triglidae {e.g. the 

 Gurnards), Percidae {e.g. the Bass), and Clupeidae like the Pil- 

 chard and Sprat, but not the Herring, whose adhesive demersal 

 eggs are deposited in clumps on shingly banks in the sea at 

 varying distances from the shore. 



The eggs of Elasmobranchs are deposited singly or in pairs at 



Fig. 235. — Diagrams of the pelagic ova of a Cod or a Plaice (A) and of a Ling (Molva). 

 O, Germinal disc ; M, micropyle ; O.G, oil globule ; Y, yolk. (From Cunningham.) 



considerable intervals, and the period of egg-laying is prolonged 

 over a considerable part of the year. In most other Fishes, as in 

 Teleosts, the period of spawning is- limited to a few months, 

 usually in the spring and summer in temperate latitudes ; and in 

 the case of a single Fish it may last only a few days or weeks, 

 but the number of eggs produced is often enormous. Thus, in a 

 Ling 61 inches long and weighing 54 pounds the ovaries con- 

 tained 28,361,000 eggs. A Turbot, 17 pounds in weight, had 

 9,161,000 eggs; and a Cod of 21^ pounds 6,652,000. The 

 least prolific of the British food Fishes is the Herring, in which 

 the number of ovarian eggs varied from 21,000 to 47,000 in four 

 specimens examined.^ The extraordinary fecundity of many Fishes 

 seems to bear no relation to the relative abundance of the Fishes 

 themselves, but rather it is to be associated with certain disad- 



1 Cunningham, 021. cit. p. 69. 



