DEVELOPMENT OF POLYPES AND MEDUSAE. 563 



the animal ; the outer one constituting the external integu- 

 ment, and the inner becoming the lining of the stomach. 

 The arborescent fabric of the composite Hydrozoa ( 124) 

 is gradually evolved by continuous gemmation from the original 

 Polype ; and whilst in some of them the sperm-cells and 

 ova are developed within peculiar capsules not ostensibly 

 differing (except in size) from the ordinary polype-cells, 

 there are others in which they are the product of peculiar 

 buds having the form and structure of Medusce, which buds 

 in many instances become detached, and henceforth live as 

 independent zooids, their sexual apparatus being only evolved 

 after they have separated themselves from the parent stock. 

 The sperm-cells and the ova are developed within different 

 Medusa-buds ; but both kinds of buds may (in many cases at 

 least) be put forth from the same Polype-stock, as in moncecious 

 Flowering-Plants. 



739. Although the two layers of the germinal membrane 

 remain united in the Hydra and other Zoophytes formed upon 

 its simple plan, they separate from each other at certain points 

 in the Sea Anemone and its allies, so that a series of chambers 

 is formed between them ; and these chambers are afterwards 

 set apart for the production of sperm-cells and germ-cells 

 ( 126). We do not meet in this group of Anthozoa with 

 any example of that detachment of the sexual apparatus in 

 the form of separate zooids, which is so remarkable a feature 

 of the Hydrozoa. 



740. The development of the Medusce, as elucidated by 

 recent discoveries, presents several features of extraordinary 

 interest. The sexes are distinct in these animals ; sperm- 

 cells being developed in some individuals, and ova in others, 

 within the fxrar chambers that surround the stomach ( 120). 

 When the ova have received the fertilizing influence, their 

 first products are ciliated gemmules resembling those of 

 Hydraform Polypes (fig. 304, a). These, after moving about 

 for some time in the ovarial chambers of their parent, make 

 their exit by the orifices of these, and then swim freely 

 through the water. Gradually, however, they undergo the 

 usual elongation, and fix themselves by one extremity (e) ; at 

 the opposite extremity a depression appears in the middle, 

 which is to become the mouth, as seen at 6, and an elongation 

 of the four corners (c,f) gives origin to the first tentacula, 



o o 2 



