THE CEYLON PEARL-OYSTER. 309 
The outermost layer of the periostracum (which in Margariti- 
Jera is a negligible quantity) and the hypostracum probably 
arise by direct transformation of the outermost portions of 
specialised epidermal cells, and on this account it may prove 
necessary to draw a sharper morphological distinction between 
them and the rest of the shell than has hitherto been done *. 
The prismatic layer and the nacre, together with the inner 
layers of the periostracum, more probably arise as a secretion 
which first hardens into a membrane in sitw, and then forms 
the delicate skin which Huxley observed between the mantle 
and the shell in the freshwater mussels. 
It would appear that the lime-salts and albuminous fluid 
which hardens to form the conchyolin are independent of each 
other, and may be secreted in varying proportions. Where 
these two constituents are secreted under circumstances which 
inhibit the control of the shell-secreting epidermis, or where 
the secretion takes place so copiously and rapidly that the 
epidermis is unable to regulate the deposition (as in the patho- 
logical cases described below), lime-salts are precipitated in a 
columnar form, much as in Harting’s bodies, and, concurrently 
with this, the albuminous fluid is transformed into an insoluble 
substance resembling conchyolin. The process of shell-secretion 
at the rapidly growing edge of the shell resulting in the 
formation of the prismatic layer—which in Margaritifera 
vulgaris measures aS much as 1 mm. or more in thickness—is 
probably in some degree analogous to the process of secretion of 
repair-substance, the epithelium exercising comparatively little 
control over the arrangement of the elements. 
But in the case of the nacre it is different. Here the epithe- 
lium seems to exert a definite and very strict selective influence 
resulting in the finely stratified and chambered structure which 
can, I think, best be interpreted as arising from rhythmically 
intermittent secretory action on the part of the controlling 
epidermis. Any disturbance of the normal rhythm of this 
secretion, e.g. the stimulation of an intrusive particle between 
shell and epidermis, results in the formation of the irregular 
substances described below, such as granular repair-nacre, the 
several varieties of columnar repair-substance, or the amorphous 
non-calcified substance. 
It would thus seem as though the structure of the shell- 
substance, and its variations, normal and pathological, could 
be expressed in terms of the proportions of lime-salts and organic 
* The difference between the outermost layer of the periostracum and the hypo- 
stracum on the one hand, and the remainder of the shell on the other, the former parts 
arising by direct cell-transformation or cuticularisation of cell-protoplasm, the latter 
as a secretion poured out by the cells, suggests a line of inquiry that might yield 
interesting results. Can these two constituents of the shell be separated morpho- 
logically and phylogenetically, and, if so, can the former be regarded as in any sense 
homologous with the cuticular exoskeleton of an ancestor common to Mollusca and 
Arthropoda, the latter being a subsequent addition peculiar to the Mollusea. 
associated with their more sedentary modes of life, which has now, for all practical 
purposes, replaced the more strictly cuticular element as an exoskeleton ? 
