90 LIFE IN THE SEA [PART I 



only in the case of the larger fishes and the rarer marine 

 mammalia that man's influence as a destructive agent is seriously 

 felt. As a destructive agent he has far less effect on the abundance 

 of most useful marine animals than is at first apparent. 



But no less astonishing is the incredible fecundity of marine 

 organisms and the variety of contrivances for the protection of 

 both immature and adult animals which the struggle for existence 

 has evoked. If the amount of destruction is great no less great 

 are the recuperative powers of a marine species, and indeed it is 

 only by measuring the latter that one can arrive at an estimate 

 of the amount of the destruction. A large turbot may spawn nine 

 millions, a cod five, a flounder one, a plaice three hundred 

 thousands of eggs during each breeding season. Other fishes are 

 almost as fertile. Even the relatively slowly-breeding Crustacea 

 produce great numbers of eggs : crabs, lobsters, prawns and their 

 allies produce several thousands of eggs at each spawning. Most 

 molluscs are very prolific, and though no one has attempted to 

 estimate the number of eggs annually produced by a mussel, yet a 

 glance at the enormous number of eggs that can be taken from 

 the ovary of this animal on the point of a knife will convince one 

 that it must be very great. Starfishes and their allies are very 

 fecund. Bottom-living worms often deposit their eggs in cocoons 

 and each of these may contain an astonishing number of ova. 

 Speaking quite generally benthic animals are more prolific than 

 pelagic forms, but even in the planktonic worms hundreds of eggs 

 may be carried by one individual. Pelagic Crustacea also are less 

 fertile than bottom-living forms, but they have advantages in the 

 more abundant opportunities for the distribution of their eggs and 

 larvae. The powers of reproduction of the compound polyzoa and 

 zoophytes are astonishing. In these animals large numbers of 

 zoids are produced by the budding of an originally single indi- 

 vidual, and of these zoids, which together form the association 

 known as the polyzoon or zoophyte, a large proportion at least are 

 reproductive and produce eggs which are liberated in the sea to 

 develop into free-swimming larvae, which either settle down on 

 the sea bottom to reproduce another colony or may themselves 

 bud asexually or again produce eggs. Hardly a marine animal 

 but harbours some parasite and animal parasites above all other 



