DEVELOPMENT OF POLYPES AND MEDUSZ. 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 Meduse, 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 monecious 
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 Anthoza with 
any example of that detachment of the sexual apparatus in 
‘the form of separate zooids, which i is so remarkable a feature 
of the Hydrozoa. 
740. The development of the Medusa, 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 four 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 (¢) ; at 
the opposite extremity a depression appears in the middle, 
which is to become the mouth, as seen at }, and an elongation 
of the four corners (¢,f) gives origin to the first tentacula, 
002 
