CH. IV] LIFE IN THE SEA 89 



stage in which the animal is developing within the egg ; (2) the 

 larval stage which begins when the embryo hatches out from the 

 egg and which may be reduplicated by the development of a 

 series of larvae; (3) the juvenescent stage in which the larva has 

 attained the form of the adult and can be recognised as belonging 

 to a definite species, even though its life-history has not been 

 followed out ; and (4) the adolescent stage in which the young 

 animal has become sexually mature. This sequence is of course 

 only a general one and it is modified in all kinds of ways. 



Death as the result of senile decay must be a very exceptional 

 event in the case of a marine animal. Usually the life of such is 

 determined by some catastrophe. In the sea the struggle for 

 existence is probably more severe than on the land. Every animal 

 has its own peculiar enemies, either predatory creatures which 

 prey upon it, or parasitic organisms which in their ultimate effect 

 are no less to be feared. Physical events may cause havoc in the 

 sea. Violent alterations of temperature may lead to the death of 

 hosts of creatures, while great changes in the salinity of the sea- 

 water may be no less fatal. 



Countless millions of pteropods must be destroyed by the 

 whales of the northern seas ; porpoises destroy hosts of herring, 

 cod, whiting and other fishes ; roving sharks and dogfishes, either 

 singly or in shoals, must at times produce devastation among the 

 bottom-living fishes of sea areas ; cod which are themselves the 

 prey of porpoises devour great numbers of fish such as herrings, 

 and Crustacea such as hermit crabs, &c. ; plaice and flounders eat 

 enormous numbers of cockles, mussels and other small shellfish, 

 and densely populated beds of these molluscs are at times 

 decimated by hordes of starfishes ; pelagic fishes like herrings and 

 mackerel feed to a great extent on swarms of copepods and other 

 planktonic Crustacea, and 20 millions of Ceratium have been 

 estimated in the stomach of a single sardine. And so on through 

 the whole marine animal kingdom. Sea birds prey to an astonish- 

 ing extent upon molluscs and fishes. Finally man, the destroyer, 

 contributes his share to this incredible massacre, for whatever is 

 useful in the sea is caught by him to the full extent of his powers, 

 aided by fishing machinery ; and not only useful animals but also 

 those which are in association with these are destroyed. But it is 



