442 Natural History of British Zoophytes. 
deposition of layer over layer, secreted from without ; while those 
of the tribes in question are endogenous, and receive all their incre- 
ment in thickness, from secretions deposited from within. When 
the material is calcareous or membrano-calcareous, it is moulded 
into cells or short tubes, which are placed generally in apposition, 
and in one or two layers or series, and arranged in straight rows in 
a very regular manner, so that every cell alternates with the one 
at its side. This arrangement is very obvious in the Flustrae, but 
it is not less real in Cellepora and Eschara, in which, by the cells 
being immersed, it is partly concealed. When, on the contrary, 
the material is horny, it is formed into tubular sheaths encasing the 
living flesh, jointed at intervals, sometimes of the same calibre 
throughout, but more commonly dilated at intervals into vases or 
cups, or cells, in which the proper body of the polype is placed. 
The manner in which the sheath or tube is divided and branched, 
is limited in diversity only by the number of the species, which are 
among the most delicate and interesting of all polypidoms, and pre- 
eminently imitative of vegetable forms. These forms are of course 
altogether independent of their animated tenants, these "have been 
specifically appointed by Him to do what they have done, and are 
still effectuating. They are mere instrumentalities- at His com- 
mand. They know nothing of the results they cause, nor mean to 
perform any of them, nor could of themselves co-operate with each 
other, nor produce any systematical arrangement, or regulated or 
orderly effects. It is their Master and Maker who organizes, go- 
verns, and guides them to those movements and operations which 
they perform, and from all others ; so that by His directing will 
they are made to do what we see them effect, and that only, be- 
cause He restrains and averts them from all else." * 
The formation of polypidoms has been the subject of considerable 
discussion. The opinion of Ellis, as we have already seen, was, 
that they are the result solely of a transtidation, or excretion of the 
constituent matters from the body of the polypes, and this opinion 
has been maintained recently by Lamarck, and some other naturalists. 
It rests on the assumption that the polypidom is extravascular and 
inorganic, so that after its first solidification, it suffers no alteration 
in form and quality, beyond what is evidently effected by the ope- 
ration of chemical and mechanical causes : the changes resulting 
from its increase in size, are not from the activity and pulsion of 
any inherent principle, but from the super-imposition of addi- 
tional layers, or from the additions of new cells, or from the pro- 
* Turner's Sac. Hist, of the World, Vol. ii. p. 71. 
