132 REPRODUCTION OF HYDROID ZOOPHYTES, 
are a number of little cells or bell-shaped chambers, with — 
their mouths upwards, every one of them containing a pol . 
that bears a strong resemblance to the Hydra. Each of these — 
polypes is capable of living independently of the rest, obtains — 
its nourishment by means of its own arms, and digests it in 
its own stomach ; but all are connected by a canal that passes 
along the stem and branches, in which a kind of circulation 
takes place, that strongly reminds us of that of the compound 
Tunicata (§ 114). This plant-like structure extends itself by 
budding; new branches are formed from those previously 
existing ; and these are enlarged at a certain point into cells, 
in which after a time polypes make their appearance. 
125. Besides the cells containing the polypes, however, we 
find capsules in which are evolved buds of a different nature, 
that form within themselves the generative products. These 
buds in some instances assume the form of Meduse, and, 
becoming detached from the stalk that put them forth, swim 
about freely, living upon food obtained by themselves, and 
setting free either sperm-cells or germ-cells, by the concur- - 
rence of whose contents eggs are formed, from which new 
polype-growths arise. In other instances the Medusoid bodies 
give forth their generative products, without ever leaving the 
capsules in which they were themselves developed. And in 
other cases, again, it does not seem that any Medusoid form 
intervenes at all, the germ-cells and sperm-cells being evolved 
from the Zoophytic structure itself. But since it is also 
known that even the most characteristic Medusan forms are 
evolved as buds from a Zoophytic stock (Chap. xv.), and since 
those composite forms of Acalephz whose structure has until 
lately been most obscure, turn out to be, as regards their 
essential characters, Hydrozoa organized for floating, there 
seems to be no longer any sufficient ground for ranking the 
Acalephze as a separate class. 
126. It is not, however, by animals of this very simple 
structure, that the massive stony fabrics are built up, which 
constitute the coral islands of the Pacific Ocean, and of which 
a large portion of our limestone rocks seems to be composed. 
These are constructed by animals belonging to the group of 
Anthozoa, and formed upon the same general plan with the 
Sea-Anemone,—a plan which is higher than that of the Hydra, 
inasmuch as we find the interior of the body containing other 
