2 38 
ZOOPHYTA ASCIDIOIDA. 
order, the cell, although pre-eminently entitled to the name of 
polypidom from its appearance and use, is a living portion of 
the animal which it seemingly contains. The cell is in fact the 
outer tunic of the polype, analogous to the envelope of the com- 
pound mollusca, endowed certainly with no very sensible or active 
properties of life, yet in organic connection with the interior parts 
and liable to organic changes. The relationship in which they 
stand to one another is nearly, if not precisely, the same as 
that which the fleshy crust of the Asteroida bears to its polypes, 
as a comparison of the Alcyonium with the Alcyonidium or Al- 
cyonella will render sufficiently plain ; and it is not less real 
even in those genera where the cells, when dried, have hard 
calcareous and apparently impermeable parietes. For the proof 
of this fact, — a very important one in their physiology, and in 
any question touching their rank in the animal kingdom, — na- 
turalists are principally indebted to Mil ne-Ed wards, and I can- 
not do better than lay his arguments in its behalf before the 
reader in a translation of his own words. * The connection is 
effected by means of an inner tunic which, after enclosing the 
polype’s body as in a pouch, is afterwards reflected over the 
aperture of the cell, — the reflected portion becoming exterior 
and solidified either by calcareous depositions in its texture, or 
by a mutation of its thin membranous character into a horny 
investment better suited to the office it has now to perform of pro- 
tecting the sentient body from a too rough contact of the medium 
in which the animals live, and from worser foes. From this mode 
of connection it results that when the polypes retire within, they 
at the same time must close the aperture to their cells, for that 
portion of the inner tunic which is pushed outwards by their 
exit, in their withdrawal follows the body by a process of invagi- 
nation, becoming at one and the same time a sheath for the 
column of tentacula, and a plug to the aperture, which, when 
of a flexible material, has its margins also drawn tighter and 
closer together. 
The polype which endues itself with this cell is widely differ- 
ent from any previously described ; and in a system that should 
pretend to arrange animals according to their agreements in or- 
ganization, could not be placed in one common class. Be- 
* See Additional Note, No. 4. 
