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"The Preservation of Natural History Specimens. I. Invertebrates. R. Wagstaff and J. H. Fidler. 1961."
I have found this to be a most authoratative text.
pp. 171-172: "Formalin - this is the most useful preservative. It is slightly acid and may damage specimens containing calcareous matter. it may be neutralized by being shaken with powdered chalk (5 gm/litre) and then filtered. Borax may also be used for the same purpose.
If formalin is neutralized the colours of specimens are generally better preserved than when in alcohol.
Use at 3-10% for storage.
As a preservative formalin is to be recommended for the more delicate soft-bodied animals as if used in the correct proportions it does not cause shrinkage"

I (TT) used to use formalin to 'fix' animal tissue prior to section cutting on a microtome. Tissue and cells were perfectly preserved; not sure what Jorge means by "it will damage the structures of the specimens"