An animal recently discovered, Dicyema, may at this initial stage of our inquiry be left with its place and affinities undetermined. It is a minute worm-like creature of most exceptionally simple structure, which lives parasitically within cuttle-fishes.
We now pass to animals (if so they are really to be considered) which are the lowest and simplest of all, and which are mostly microscopic in size, and may be grouped together under the term HYPOZOA, or under the generally employed name Protozoa. With very few exceptions these animals are aquatic, and if terrestrial they are found in damp localities. Some are marine, others are fresh-water organisms.
The highest of the group are the animalcules, which are named Infusoria, most of which are freely swimming organisms, though a certain number of them live fixed to some supporting body.
Another group of Hypozoa is that termed Gregarinida, a group made up of very lowly parasites, such as are often found tenanting the intestines of insects as well as those of higher animals. Finally, we have the group of Rhizopoda, animals which have the faculty of projecting 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 Amœba, are naked, and the simplest of all animals, Protogenes and Protamœba, 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.