352 



ANIMAL LIFE-HISTORIES 



colonies of Hydroid Zoophytes. But if this view approximates 

 to the truth, it is rather difficult to understand why some of the 

 Hydroid Zoophytes have given up the jelly-fish stage, which 

 presumably increased their chances in the struggle for existence. 

 It can only be conjectured that since the production of eggs from 

 which free-swimming larva hatch out takes less time and involves 

 less expenditure of energy than the production of jelly-fishes that 

 serve the same purpose, some Zoophytes have abandoned the 

 latter plan altogether, while others are giving it up. 



All jelly-fishes are not developed as buds on a fixed colony. 

 Many of them result from the transverse fission of attached indi- 

 viduals, as, e.g., the 

 common form Aurelia 

 (fig. 870), which af- 

 fords another illus- 

 tration of alternation 

 of generations. The 

 fixed individual here 

 divides transversely 

 into a number of lit- 

 tle flat Ephyrae, each 

 of which grows into 

 an adult Medusa. 



Some of the jelly- 

 fishes related to Aurelia, and others belonging to certain other 

 groups, have suppressed one stage in the cycle of existence, 

 reminding us of some of the Hydroid Zoophytes. But in this 

 case it is the fixed or vegetative stage which has been eliminated, 

 while the egg-producing Medusa stage has been retained. In 

 these species, which include some of the large forms of jelly-fish 

 common in British seas, there is consequently a succession of 

 Medusae, instead of an alternation of these with fixed forms. 



In some of the groups already mentioned there are interesting 

 arrangements for securing the well-being of egg-cells, or, it may 

 be, embryos. In Hydra, for example, as we have seen (p. 340), 

 the single egg-cell in the ovary is the surviving member of a 

 small mass of similar cells, with which it has had literally to 

 struggle for existence. And it has not merely supplanted its 

 sister-cells, but actually eaten them, affording us an illustration 

 of cannibalism the precocity of which it would be hard to beat. 



Fig. 870. Life-history of Aurelia, enlarged, a, b, c, Stages in transverse 

 division of fixed individual; de, an Ephyra. 



