238 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 Milne-Edwards, 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 ])lug 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 diff'er- 

 ent from any previously described ; and in a system that should 

 pretend to arrange animals according to their agreements in or- 

 oanization, could not be placed in one common class. Be- 



• See Additional Note, No. 4. 



