There are enormous numbers of these single-celled animals existing in all parts of the world.
Many other forms might be cited to illustrate reproduction in single-celled animals, whether free or in colonies, but all such cases would be practically but repetitions or modifications of those we have already examined.
One of the simplest of the single-celled animals is the Ameba (Fig.
The one-celled animals are called Protozoa, and the many-celled animals Metazoa.
After a few days small one-celled animals appear; these multiply with wonderful rapidity, so that in some cases the surface of the water seems to be almost white with active one-celled forms of life.
Footnote 12: It has recently been discovered that under some conditions these bacteria are preyed upon by tiny one-celled animals (protozoa) living in the soil and are so reduced in numbers that they cannot do their work effectively.
The phenomenon of repeated division of the germ-cell is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals.
The simplest of all fish like vertebrates, the lancelet (Branchiostoma) has very small eggs, and in their early development it passes through stages that are typical for all many-celled animals.
This third well-marked stage is called the gastrula stage; and it is thought to occur either typically or in some modified form in the development of all metazoa, or many-celled animals.
This stage also is common to all the many-celled animals.
Some of these Protozoans attain a remarkable size, instead of being microscopic, as is the case typically with the one-celled animals.
We have seen that our story began with the One-celled Animals, and went on with the tale of the Two-layered Animals, in which each layer was built up by cells in partnership.
We now reach the realm of the true many-celled animals, or Metazoa, where the biological units are combined to form an organic association displaying many more resemblances to a human society.
Their structural plans are far more varied, and they range more widely from higher and relatively complicated organisms to the unitary one-celled animals.
Without leaving the group of one-celled animals typified by Amoeba, we find colonies of the most elementary biological nature, where other natural obligations are added to the two of greatest importance.
The most ancient single-celled animals, being naked cells, could admit solid particles into the interior of their soft bodies, as do the Amoebae (Fig.
The first single-celled animals of this kind arose out of Monera by the differentiation of the inner kernel and the external protoplasm; they lived in the earlier Primordial period.
Oral groove, a funnel-shaped groove in one side of some one-celled animals, conducting food to the mouth.
Comparative Study of Protozoa To enlarge your idea of what a cell can do, spend as much more time on the one-celled animals as your course will permit.
Body wall, the outer wall in bodies of the many-celled animals.
Food balls, bits of food inside the cells of many one-celled animals, usually showing through the walls.
The evolution of the plant I reserve for a later chapter, and I must be content to suggest the development of one-celled animals on very broad lines.
It is well known that the horny texture to which we commonly give the name of sponge is the former tenement and shelter of a colony of one-celled animals, which are the real Sponges.
We have still many loose associations of one-celled animals in nature, illustrating the approach to a community life.
The Coelenterates, which comprise the simplest of the many-celled animals, are saclike forms with mouth openings, but with few other differentiations of parts.
Reading downward in this classification, the degree of complexity of organization steadily increases from single-celled animals to man himself.
The above list will hopefully provide you with a few useful examples demonstrating the appropriate usage of "celled animals" in a variety of sentences. We hope that you will now be able to make sentences using this group of words.
