United States or Cameroon ? Vote for the TOP Country of the Week !
There is not known to be a single ordinal form of insect extinct. There are only two orders extinct among the Crustacea. There is not known to be an extinct order of these creatures, the parasitic and other worms; but there are two, not to say three, absolutely extinct orders of this class, the Echinodermata; out of all the orders of the Coelenterata and Protozoa only one, the Rugose Corals.
In the third place, we find that beyond these two great antagonistic agencies, modern Geology recognizes sundry minor ones: those of glaciers and icebergs, those of coral-polypes; those of Protozoa having siliceous or calcareous shells each of which agencies, insignificant as it seems, is found capable of slowly working terrestrial changes of considerable magnitude.
Sexual conjugation thus remains a process which is radically the same as the non-sexual mode of propagation which preceded it. The fusion of the nuclei of the two cells was regarded by Van Beneden, who in 1875 first accurately described it, as a process of conjugation comparable to that of the protozoa and the protophyta.
As an individual Hydra may be regarded as a group of Protozoa which have become partially metamorphosed into different organs; so a Physalia is, morphologically considered, a group of Hydræ of which the individuals have been variously transformed to fit them for various functions. This differentiation upon differentiation is just what takes place during the evolution of a civilized society.
Surely the scientific mind of an age which contemplates the solar system as evolved from a common revolving fluid mass which, through experimental research, has come to regard light, heat, electricity, magnetism, chemical affinity, and mechanical power as varieties or derivative and convertible forms of one force, instead of independent species which has brought the so-called elementary kinds of matter, such as the metals, into kindred groups, and pertinently raised the question, whether the members of each group may not be mere varieties of one species and which speculates steadily in the direction of the ultimate unity of matter, of a sort of prototype or simple element which may be to the ordinary species of matter what the Protozoa or what the component cells of an organism are to the higher sorts of animals and plants the mind of such an age cannot be expected to let the old belief about species pass unquestioned.
The last class, the Sporozoa, or the spore-forming animals, while small in the number of known species, only about three hundred kinds being known, is extremely important. A number of diseases in man and other animals are due to the presence of these Sporozoans, for they are all parasitic. Few if any animals are exempt from their attacks. They even attack other minute Protozoa.
Comparative anatomy has asserted that this is so, as we have already learned, for it finds that adult animals array themselves at different levels of a scale beginning at the bottom with the protozoa, continuing on to the two-layered animals like Hydra and jellyfish and sea-anemones, and then extending upwards to the region of the more complicated invertebrates and vertebrates.
The image of a tree is misleading from the fact that all the different branches of the animal kingdom, from the protozoa up to man, have come along with what we call the higher branches, the mammals; the suckers have kept pace with the main stalk, so that we have the image of a sheaf of branches starting from a common origin and all of equal length.
They belong to various groups both of the Protophyta and Protozoa. To have some idea of those ancestors of our race that succeeded phylogenetically to the Moraeada, we have only to follow the further embryonic development of the morula. In this way the solid mulberry-embryo becomes a hollow sphere, the wall of which is composed of a single layer of cells.
And by tissues we mean groups of cells modified in form and structure for the performance of a special work or function. The protozoa developed the cell for all time to come, the coelenterata developed the tissues which still compose our bodies. But they had them mainly in a diffuse form. A sort of digestive and reproductive system they did possess.
Word Of The Day