§2 HISTORY OF BRITISH ZOOPHYTES. 
but the joints are without hinges. The stem and branches 
are of the same material. Along the sides or at the upper 
extremities of the branches we find the cup-lke cells of 
the polypes. Intermingled with the cells there are other 
vessels, called vesicles, formed of the same material as the 
cells, but widely distinguished from them by their larger 
size and by their different shape. Those vesicles contain 
the ovules from which another generation of polypes is to 
spring. 
The polypidoms, when dried, are generally of a yellowish- 
horny colour. The substance of which they are formed 
seems analogous to horn. In a young state it adheres in 
some degree to the pulpy substance of the animal, but it 
afterwards become detached in consequence of its shrivelling, 
and also in consequence of the movements of the animal 
that it contains. Some distinguished naturalists argue that 
this horny sheath is vascular and organized, and conclude 
from this that the polypidom has a growth of its own inde- 
pendent of the animal that inhabits it. But other natu- 
ralists, no less distinguished, maintain, and apparently with 
greater truth, that there is but one hfe and one plan of de- 
velopment in the whole mass, and that this depends, not on 
the polypi, which often fall off, as in Zudularia, but on the 
