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behave in this respect much like a colony of amœbæ. The cells of both layers have at their bases long muscular fibrils, those of the ectodermal cells running longitudinally, those of the entoderm transversely. The animal can thus contract its body in both directions, or, if the body contain water and the transverse muscles are contracted, the pressure of the water lengthens the body and tends to extend the tentacles.
cells and fibrils.
On the outside of the elastic membrane, just beneath the ectoderm, is a plexus or cobweb of nervous As in every nervous system, three elements are here to be found. 1. An afferent or sensory nerve-fibril, which under adequate stimulus is set in vibration by some cell of the epidermis or ectoderm, which is therefore called a sensory cell. 2. A central translates it into consciousness, and is the seat of or ganglion cell, which receives the sensory impulse, whatever powers of perception, thought, or will the This also gives rise to the effer
motor fibril to the corresponding muscle, exciting its ent or motor impulses, which are conveyed by (3) ing the different ganglion cells, so that they may act in contraction. But there are also nerve-fibrils connectunison. In the higher animals we shall find these central or ganglion cells condensed in one or a few masses or ganglia. But here they are scattered over the whole surface of the elastic supporting membrane.
The reproductive organs for the production of eggs
on the out
and spermatozoa form little protuberances
side of the body below the tentacles. But hydra reproduces mostly by budding; new individuals growthe trunk of a tree, but afterward breaking free and ing out of the side of the old one, like branches from
leading an independent life. There are special forms of cells besides those described; nettle cells for capturing food, interstitial cells, etc., but these do not con
The distance from the single-celled amoeba to hydra is vast, probably really greater than that between any other successive terms of our series. It may therefore be useful to consider one or two intermediate forms and the parallel embryonic stages of higher animals, and to see how the higher many-celled animal originates from the unicellular stage.
The amoeba is an illustration of a great kingdom of similar, practically unicellular forms, which have played no unimportant part in the geological history of the globe. These are the protozoa. They include, first of all, the foraminifera, which usually have shells composed of carbonate of lime. These shells, settling to the bottom of the ocean, have accumulated in vast beds, and when compacted and raised above the surface, form chalk, limestone, or marble, according to the degree and mode of their hardening.
The protozoa include also the flagellata, a great, very poorly defined mass of forms occupying the boundary between the plant and animal kingdoms. They are usually unicellular, and their protoplasm is surrounded by a thin, structureless membrane. This prevents their putting out pseudopodia as organs of Instead of these they have at one end of the ovoid or pear-shaped body a long, whiplash-like process or thread, a flagellum, and by swinging this they propel themselves through the water. These flagellata seem to have a rather marked tendency to form colonies. The first individual gives rise to others by
WHENCE AND THE WHITHER OF MAN individuals remain connected by the undivided rear But the division is not complete; the new end of the body. And such a colony may come to contain a large number of individuals.
Such a colony is represented by magosphæra. This is a microscopic globular form, discovered by Professor Haeckel on the coast of Norway. It consists of a
large number of conical or pearshaped individual cells, whose apices are turned toward the centre of the sphere. The cells are cemented together by a mucilaginous substance. Around their exposed larger ends, which form the surface of the sphere, are rows of flagel
la, by whose united action the colony rolls through the water. After a time each individual absorbs its fla
gella, the colony is broken up, the different individuals settle to the bottom, and each gives rise by division to a new colony. This group of cells may be considered or as an individual. Each term is de
as a colony fensible.
2. MAGOSPHERA PLANULA.
Volvox is also a spheroidal organism, composed often of a very large number of flagellated cells. differs from magosphæra in certain important respects. In the first place its cells have chlorophyl, the
coloring matter of plants. It lives therefore on un-
The cells of volvox do not form a solid mass, but have arranged themselves in a single layer on the outer surface of the sphere. For a time, under favora ble circumstances, volvox reproduces very much like magosphæra, and each cell can give rise to a new, many-celled individual. But after a time, especially under unfavorable circumstances, a new mode of reproduction appears. Certain cells withdraw from the outer layer into the interior of the colony. Here they are nourished by the other cells and develop into true reproductive elements, eggs and spermatozoa. Fertilization, that is, the union of egg and spermatozoon, or mainly of their nuclei, takes place; and the fertilized egg develops into a new organism. But the other cells, which have been all the time nourishing these, seem now to lack nutriment, strength, or vitality to give rise to a new colony. They die.
We find thus in volvox division of labor and corresponding difference of structure or differentiation; certain cells retain the power of fusing with other cor
are evidently imNatural death cannot touch
responding cells, and thus of rejuvenescence and of giving rise to a new organism. And these cells, forming a series through all generations, a mortal like the protozoa. them. These are the reproductive cells. The other cells nourish and transport them and carry on the work of excretion and respiration. These latter correspond practically to our whole body. We call them somatic entirely subservient to, and
cells. In volvox they are
exist for, the reproductive cells, and die when they have completed their service of these. The body is here Furthermore, in volvox there
only a vehicle for ova. has arisen such an interdependence of cells that colony. The colony we can no longer speak of it as a
has become an individual by division of labor and the
resulting differentiation in structure.
special work or
But hydra gives us but a poor idea of the cœlenterata, to which kingdom it belongs. The higher cœlenterata have nearly or quite all the tissues of higher animals-muscular, connective, glandular, etc. And by tissues we mean groups of cells modified in form and structure for the performance of a function. The protozoa developed the cell for all time to come, the coelenterata developed the tissues which still compose our bodies. mainly in a diffuse form. A sort of digestive and reproductive system they did possess. But the work of arranging these tissues and condensing them into compact organs was to be done by the next higher group,
But they had them
Let us now take a glance at certain stages of em
bryonic development which correspond to these earli
est ancestral forms. We should expect some such