564 
DEVELOPMENT OF MEDUS2E. 
which are afterwards increased by the addition of many others 
(g). In this manner a true polype is formed, which leads the 
life of a Hydra, and, like it, propagates its kind by the forma¬ 
tion of polype-buds, which detach themselves and lead inde¬ 
pendent lives ; and thus from a single Medusan egg there may 
arise a whole colony of polypes multiplied by gemmation. These 
differ entirely from Hydrse, however, in regard to their sexual 
apparatus, which is detached (as in the 0 composite Hydrozoa) 
under an entirely different form, that of a Medusa. The body of 
the polype undergoes a great 
lengthening, and seems as if 
divided by transverse bands, 
which gradually deepen, so as 
to make the whole body almost 
resemble a pile of saucers 
with divided edges (h); for 
beneath the lowest of these 
saucer-like disks, a new set 
of polype - arms makes its 
appearance; and after the 
detachment of the whole pile of disks, the polype-body 
remains at their base, and may continue to lead its former 
life, and to propagate itself in the polype-form. The disks 
progressively enlarge, those at the summit of the pile in¬ 
creasing most rapidly, and then detaching themselves from 
the pile (i); when thus detached, they swim about freely in 
the water after the manner of the smaller and simpler Me¬ 
dusae, to which they closely correspond in form (d) ; and they 
gradually enlarge and acquire the structure of their original 
parents (7c). It is not correct to represent (which is commonly 
done) the pile of Medusa-disks as being formed by the sub¬ 
division of the polype-body. The Medusa-disks are in reality 
sexual buds, resembling those of the composite Hydrozoa 
(§ 125); and the only essential difference between the two 
cases lies in the fact, that among the latter it is the Zoophytic 
form which ostensibly constitutes the animal (the Medusan 
buds being thrown-off only at certain times for a special pur¬ 
pose), whilst the former are only known (save to such as 
search-out the history of their polypoid development) in the 
Medusan stage of their lives. 
741. The recent researches of Professor Muller and others 
Fig. 304. —Development of Medusae. 
