132 FORMATION OF SHELLS OF ANIMALS, ETC., 
of matter exposed to its full influence their globular form, 
any assumed vital interference, would be superfluous and 
inconsistent. Dr. Carpenter observes, in reference to one 
form of shell-structure, the prismatic cellular substance, 
“that such is its resemblance to mucous membrane that it 
may be considered in the light of a calcified epithelium.” 
With this resemblance in structure, no doubt Dr. Carpenter 
would include the idea of identity of the forces employed 
in their formation. There are, however, two other struc- 
tures, one hard and the other soft, whose resemblance 
is more striking than that named by Dr. Carpenter, which 
I will briefly consider. One is the common integument of 
mammals; the other the dermic part of the shell of crusta- 
ceans. In both there is a deep-seated layer of membrane 
—basement membrane—in contact, by its superficial sur- 
face, with rounded particles of various sizes; next above is a 
layer of globular bodies containing granules of different 
forms and magnitudes ; and, lastly, a covering of scales 
or lamin, this depending upon circumstances. Now, 
organs with so many points of resemblance in structure, 
and both performing the same function, would be ano- 
malous, if they were formed upon an entirely different 
plan, and under the influence of agencies of a diametrically 
opposite character ; that is to say, if the component ele- 
ments of the former should owe their globular form to a 
vital force residig in these globules, whilst it has been 
demonstrated, that the elements of the latter owe their 
arrangement in the same form to a force entirely phy- 
sical; and the more especially so, as there is no other dif- 
ference in these tegumentary structures, but that occa- 
sioned by the presence of carbonate of lime im the one, 
and its absence from the other. However anomalous it 
may appear that two such organs as those just mentioned 
EE 
