ing and retracting (so to say, at will) filamentary or conical
processes of their semi-fluid substance, such processes being the
_Pseudopodia_, which were referred to earlier.[17]

Amongst the _Rhizopoda_, the most complex and beautiful are the delicate
and symmetrical creatures known as _Radiolaria_,[18] the siliceous
skeletons of which are amongst the most remarkable of microscopic
objects.

Allied to them are the simpler _Heliozoa_, of which the after-mentioned
_Actinophrys_ may be taken as a type.

Next come the _Flagellata_, or minute creatures which swim about by
means of one or two whip-like processes, whence the name of the group.

Last of all is the group of _Foraminifera_, animals which are well
worthy of note, seeing that, though they are each but as it were a
minute particle of structureless jelly, they manage to build most
complexly-formed, generally calcareous, shells, or to pick up from the
sand of the sea minute particles, which they agglutinate around them
with marvellous neatness and precision. Their calcareous shells are
generally pierced by a multitude of minute pores, through which the
little creatures protrude their _pseudopodia_. It is from these pores
(or _foramina_) that the group receives its name. All _Foraminifera_,
however, are not provided with shells. Some, as the _Amoeba_, are
naked, and the simplest of all animals, _Protogenes_ and _Protamoeba_,
consist of but a minute particle of semi-fluid jelly, or protoplasm,
naked and as devoid of every external protection as it is of internal
organization.

We have thus descended to the bottom of the animal kingdom, and passing
from these rudimentary forms, which are generally reckoned as animals,
we may next survey in ascending order the different organisms which
together compose the kingdom of Plants, a group much less rich in
species than is the animal kingdom.

At the bottom of that kingdom are very simple creatures, but little
different, to all appearance, from the lowest animals. As an example of
such we may take the minute plant _Protococcus_, which is an humble
member of the great group of _Algae_, to which all sea-weeds belong. Not
all of this important tribe, however, are marine. Many are found in
fresh water--such as the protococcus itself, and many of the green
vegetable threads known as _Conferrae_. Some even live on land, and draw
their moisture from the atmosphere. The _Algae_ are exceedingly varied in
their structure; some, l