78 FORMATION OF SHELLS OF ANIMALS, ETC., 
physiological explanation, is so strong, that I cannot help 
believing that, if only it had been earlier known that 
bodies exactly like these calculi could be prepared arti- 
ficially, and also all the conditions known under which 
the process is conducted, the opinion that they are orga- 
nized structures would never have been started; and 
therefore, if my view is the correct one, as I have no doubt 
it is, I claim no credit on that account, or attribute any 
want of penetration to those who have entertained a dif- 
ferent one, seeing that I commenced the investigation of 
this subject under advantage which they had not. 
The next subject which I shall investigate will be the 
structure and development of the shells of animals. 
I shall make choice for examination of those which are 
most common, and therefore can be most easily obtained. 
I shall begin with the external skeleton, or, as it is com- 
monly called, the shell of crustaceans, and, first, shall 
consider those membranous parts which are connected 
with or which make up part of the shell, but only so far 
as the process of calcification or the development of shell- 
tissue is concerned. I shall then examine more particu- 
larly the arrangement of the different structures, as they 
are found in the different parts of the completely formed 
shells of this class of animals. 
All the parts of the ordinary crustaceans, as, for instance, 
the lobster, which contain a large quantity of earthy 
matter in their composition, cannot be considered as 
pertaining to the skeleton, the carapax and some other 
parts being more analogous to integument in the office 
which they have to fill, and the parts called apodemata, 
affording attachment to muscular fibres, perform the 
function of bone, whilst others similarly related to muscle 
correspond to the osseous tendons of birds. 
