CALCIC GLANDS. 
323 
111 some genera this transitorily developed primitive shell soon 
disappears and the shell-sac becomes everted, the thickened margins 
forming the collar of the mantle and with the enclosed area secreting 
the permanent shell. 
Other groups, as Linicur, Clausilia, Vivipara, Neritina, etc., how- 
ever retain this embryonal vestige or Protoconch upon the apex of 
the permanent shell of later life. In the Limaces the invaginate 
gland closes up and the chitinoiis particle forms the nucleus of the 
permanent shell, which is secreted by the enclosing walls. In the 
Clausilia the minute vestigial shell is also enclosed and completely 
invested by the epithelium, which, however, is soon ruptured and the 
protoconch becomes partially external, the permanent shell being 
formed in continuity with it. 
In the Pelecypoda a primitive shell, the Protostracum, is also 
developed early in embryonal life, and is followed by a Prodissoconch 
stage, the vestiges of which are frequently observed upon the umbones 
of immature shells. The conchal gland appears opposite the blasto- 
pore or orifice of embryonal invagination, as a saddle-shaped group of 
epithelial cells, secreting a uniform saddle-shaped film, which later 
becomes calcified laterally to form the paired valves, the uncalcified 
median part of the pellicle uniting the valves is placed over the median 
groove and becomes the ligament. 
The Calcic Glands (calx, lime) are numerously distributed over 
the whole mantle-surface, but most richly developed along the mantle- 
margin in the shell-bearing species, and contributing to form the 
shell. Lime cells are also numerous in the digestive gland, their pro- 
duct being affirmed to be eventually largely utilized by the Gastropods 
in the production of the calcareous epiphragm ; they are also 
distributed over the whole external surface of the body, especially in 
tho.se species destitute of definite shells, calcareous granules or spicula 
being deposited in the skin, and probably contributing to the future 
development of a new shelly covering or protection for the body, to 
replace that now practically lost by degeneration, and constituting a 
renewal of the cycle of development of an external shell, such as has 
taken place more than once in the past, as evidenced by the transitory 
vestiges present temporarily in the early larval stages of many species 
and therefore preceding the shell they now possess. 
Lime cells are also freely distributed throughout the general 
tissues of the body, their action under suitable conditions producing 
