96 FORMATION OF SHELLS OF ANIMALS, ETC., 
under which they are acted upon by the physical forces 
during the different stages of their elaboration into per- 
fectly formed shells; just as, in the artificial process, 
vitality displayed in an act of the will first brings together 
the material substances necessary to form the globules of 
carbonate of lime, then proportions them, and, lastly, 
arranges the conditions under which these materials are 
required to be acted upon by the physical forces. There 
is, however, probably this difference, that the latter is the 
result of distinct and separate mental acts, whilst the 
former is all comprised in one act, which embraces also 
all the operations of nature. 
The preceding observations, in reference to the chemical 
conditions under which the globules of carbonate of lime 
are formed, apply entirely to the dermic portion of the 
shells of crustaceans, and especially to those parts where 
the globules are formed entirely out of the reach of the 
blood, as close to their external surface, or around the 
passages extending from this surface to the pigment 
membrane. But there are other parts of these shells, as 
the septa (apodemata) between the cavities contaming the 
muscles and those parts of the shell corresponding to the 
osseous tendons of birds, where these conditions cannot be 
demonstrated. - In these situations, the shell bemg im- 
bedded in the substance of the muscles, and giving attach- 
ment to their fibres, corresponds im function more to the 
endo-skeleton or bone than to the exo-skeleton. Now in 
these parts of the shell the globules of carbonate of lime 
are formed exactly as in the other parts, and as in the 
artificial process. Nowhere can all the stages of molecular 
coalescence, from the minutest particles up to the most 
perfectly formed globules, be better seen than in these 
septa; a fact indicating an undoubted identity of process 
