Some of the simplest foraminifers possess only one hole in the shell,
and, consequently, are enabled to throw off pseudopods from one side of
the body only. In others, of a much more complex nature, the new
chambers form a spiral in such a manner that they overlap and entirely
conceal those previously built; and the development may proceed until a
comparatively large discoid shell is the result. This is the case with
_Nummulites_, so called on account of the fancied resemblance to coins.
Further, some species of foraminifera produce a skeleton that is horny
in character, instead of being calcareous, while others are protected
merely by grains of sand or particles of other solid matter that adhere
to the surface of their glutinous bodies.