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2. That in certain other species ( Campanularia Jlexuosa, &c.) the 
final development of the ovum, previously to liberation, takes place 
in a peculiarly formed medusa-like extra-capsular sporosac, which, 
however, must not be confounded with a true medusal zooid. It 
originates as an ordinary intra-capsular sporosac, and subsequently 
becomes extra-capsular by the elongation of the blastostyle. 
The most important steps in the development of the true medusal 
buds are the following : — - 
(1.) The formation of a hollow process from the coenosarc or blas- 
tostyle. (2.) The differentiation in this process, of a peripheral por- 
tion (ectotheque), and a central portion (manubruim) ; the latter 
becoming further differentiated into two layers, ectoderm and endo- 
derm. (3.) The emission from the hollow central portion of four 
coecal tubes (radiating canals), composed of ectoderm and endoderm, 
which, simultaneously with their elongation, have their ectodermal 
layer expanded into a web-like membrane (umbrella), connecting the 
tubes laterally with one another. (4.) The further connection of 
the radiating tubes with one another by lateral branches (circular 
canal), and the development of a velum from the margin of the um- 
brella. 
From the above processes, it follows that the development of the 
medusa admits of an easy comparison with that of the alimentary 
polype, the radiating canals being developed from the central manu- 
brium in the medusa, exactly as the tentacula are from the body of 
the polype ; but that, while in the polype they continue free, in the 
medusa they are united laterally by an extension of the ectoderm 
and by lateral branches.* 
A comparison of the two kinds of reproductive zooids (sporosacs 
and medusa) and of the alimentary zooid (polype) results in the fol- 
lowing parallelism : — 
* In the remarkable species, Laomedea acuminata (Alder), the tentacles of the 
polype are actually united, as Mr Alder has shown, for a considerable distance 
from their origin, by an intervening membrane. 
