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 _Algæ_, 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 _Conferræ_. Some even live on land, and draw their moisture from the atmosphere. The _Algæ_ are exceedingly varied in their structure; some, like the protococcus, being of extreme simplicity; others attaining a large size, and presenting the appearance of a stout stem with branches and leaves.