THE GENERAL CHARACTERISTICS OF SHORE ANIMALS. 17 



on the shore is the life-history of the common animals, 

 and the peculiarities of the young forms. In some cases, 

 as in the sea-firs or zoophytes, there is what is known as 

 alternation of generations, that is the occurrence in one 

 life -history of two or 

 more different forms, 

 differently produced. 

 Thus, the sessile sea-fir 

 buds off a little swim- 

 ming-bell or tiny jelly- 

 fish, which produces 

 the eggs from which 

 new sea-firs arise. As 

 the swimming-bells can 

 move actively through 

 the water, and are also 

 very readily swept along 

 by currents, it must 

 often happen that the 

 eggs are deposited some 

 distance away from the 

 original sea-fir colony. 

 Most worms produce 

 eggs which give rise to 

 minute top-shaped Iarva3, 

 which live near the sur- 

 face of the water and 

 ensure the distribution 

 of the species. Even 

 the sluggish Echino- 

 derms, the starfish, sea- 

 urchins, and brittle-stars, 

 produce minute active 

 larvae, which present 

 no apparent resemblance 

 to the adult, and are 

 adapted for quite a 

 different kind of life. 

 But it is among the Crustacea that we have the most 

 complex and interesting life-histories. In them there is 

 not merely one peculiar larval form, but the young undergo 



Fio. 6. Swimming-bell (Sarsia) of a sea-fir, 

 showing the long tongue, or manubrium, 

 swollen by the contained eggs, and the 

 four long tentacles which bear stinging- 

 cells. After Allman. 



