RHIZOPODA. 37 
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all cases the power of secreting a skeleton or shell, which is 
technically called the “test” (Lat. zesta, a shell). The shell is 
usually “calcareous "—that is to say, composed of carbonate of 
lime; but it is sometimes “ arenaceous,” or composed of par- 
ticles of sand united together firmly by an unknown animal 
cement, and it is in some cases simply membranous. In either 
case, the shell may exhibit one or other of two very distinct 
types of structure. In the one type (as in A/zliola, fig. 6, 0), 
the shell-walls are not perforated with holes, and the pseudo- 
podia are therefore all emitted from the mouth or “oral aper- 
ture” of the shell. In the other type (as in Discordina, fig. 6, 
c), the shell-walls are perforated with a number of little aper- 
tures or ‘‘foramina,” from which the order derives its name. 
These foramina are the mouths of tubes which pierce the walls 
of the shell, and thus establish a free communication between 
the interior and exterior. In this way the sarcode which fills 
the inside of the shell is enabled to reach the outer surface, so 
as to form a film, from any part of which the pseudopodia may 
be given off. The presence or absence of foramina is believed 
to constitute a true structural distinction, and the Horaminifera 
may be thereby divided into two great and natural groups 
(Perforata and Imperforata). 
According to the form of the shell, also, the Foraminifera may 
be conveniently, though arbitrarily, divided into two sections. 
The simplest form of shell is seen in such an example as Lagena 
(fig. 6, a), where the shell consists of but a single chamber; and 
the animal, in fact, is nothing more than a little mass of sarcode, 
surrounded by a calcareous envelope. Lagena, then, may be 
taken as the type of what are called the “monothalamous” 
foraminifera (Gr. monos, single; ¢thalamos, a chamber,)—that is 
to say, of those forms in which the animal consists of a single 
segment, and the shell of asingle chamber. All the Foramint- 
Jera without exception commence life as “ simple” or “ mono- 
thalamous” forms, like Lagena, but it is comparatively seldom 
that they retain this simplicity throughout life. In the great 
majority of cases the primitive mass of sarcode, or “ primordial 
segment,” commences a process of budding, or “ gemmation” 
(Lat. gemma, a bud), by which it becomes converted from a 
“simple” into a “compound” form. The original sarcode- 
mass, that is to say, begins to throw out buds in some determin- 
ate direction ; all the buds thus produced remaining connected 
