232 APPENDIX. 
individuals in one common life, and grows mm a snrub- 
like form upon any submarine body, such as a shell, 
a rock, a weed, or even another polypidom to which 
it is parasitically attached. Each polype, in both 
classes, protrudes from and retreats within its cell by 
an independent action, and when protruded puts forth 
a circle of tentacles whose motion round the mouth 
is the means of securing nourishment. There are, 
however, peculiarities in the structure of the Poly- 
zoa which seem to remove them from Zoophytology 
to a place in the system of nature more nearly con- 
nected with Molluscan types. Some of them come 
so near to the compound ascidians that they have 
been termed, as an order, “ Zoophyta ascidioida.” 
The simplest form of polype is that of a fleshy 
bag open at one end, surmounted by a circle of con 
tractile threads or fingers called tentacles. The plate 
shows, on a very minute scale, at figs. 1, 3, and 6, 
several of these little polypiform bodies protruding 
from their cells. But the Hydra or Fresh-water 
Polype has no cell, and is quite unconnected with 
any root thread. or with other individuals of the 
—= eh CUC 
