Chap, xiv.] DEVELOPMENT OF AURELIA. 



54i 



B 



become produced into eight lobes into which prolonga- 

 tions of the central cavity extend. After undergoing 

 some further development, each saucer in turn breaks 

 away from the common stock, and, as an Ephyra, 

 with a disc of gelatinous tissue, a layer of muscle, 

 and eight bifid tentacular lobes, swims about freely, 

 increases in size, and becomes gradually converted 

 into an adult sexually 

 mature jelly-fish. Here, 

 then, we have an ex- 

 ample of "alternation 

 of generations " ; the 

 fertilised ovum gives 

 rise, through the plan- 

 ula, to the Hydra-tuba, 

 the parts of which 

 undergo by constriction 

 a serial multiplication, 

 and each part gives rise 

 to a sexually mature 

 form. 



Alternation of 

 generations. This 

 complex process has, 

 from various causes, 

 been considerably ob- 

 scured, and various 



terms have been applied to the various ways in which 

 this phenomenon has been observed. As seen among 

 the hydrozoic Crelenterates, Annelid worms, and Tuni- 

 cata, it may be thus described in the words of Balfour : 

 "The simplest cases are those in which an individual 

 which produces by sexual means gives origin to asexual 

 individuals differently organised to itself, which pro- 

 duce, by budding, the original sexual form, and so com- 

 plete a cycle .... In all these cases the origin of 

 the phenomenon is easily understood. It appears, as is 



Fig. 225. Development of Aurelia 

 aurita. 



A, Polyp stage ; B, commencement of trans- 

 verse cleavage ; c, completion of the 

 same so-called Hydra-tube stage. (After 

 Sars.) 



