A first book in organic evolution

by D. Kerfoot Shute

The flagellum is a flattened, whip-like filament, having a striated appearance, and gradually tapers from the base to its extremity. It slowly bends over five or six times a minute to the mouth, and then, more slowly still, bends away again. It is through the movements of the flagellum that particles of food are driven into the mouth and down the œsophagus into the central protoplasmic mass. In this mass and its extensions the food is digested and assimilated.

This little one-celled animal has the power, through its special chemical activities, of manufacturing and emitting light. It is through the agency of myriads of these little creatures that the diffused luminosity of some seas is produced and can be observed at night. The Noctiluca is very transparent, and for this reason it is a particularly favorable subject for the study of its luminosity or phosphorescence. They can be obtained by the tow-net in unlimited quantities from the sea and transferred into a jar of sea water. Here they soon rise to the surface, forming a thick layer. If the jar be placed in the dark and agitated in the slightest degree, there is an instantaneous display of light, which is of a beautiful greenish tint. The light emitted by the Noctiluca is so vivid that it can even be observed in ordinary lamp-light. This phosphorescence is only of an instant’s duration, and a short rest is necessary for its renewal. The special locality for the formation of the phosphorescence is in the very fine protoplasmic network, which lines the external structureless membrane or cell wall. These wonderful little Noctilucæ may well be figuratively called the fire-flies of the ocean.

Fig. 7.—Gromia oviformis with protoplasmic threads (pseudopodia) extended and forming an elaborate network in which a captured unicellular organism is seen; d, diatom captured; p, protoplasm containing captured diatoms; s, shell.

Gromia oviformis ([Fig. 7]) is often found in fresh water adhering to confervæ and other plants of running streams. The protoplasmic body of this animal is enveloped in a chitinous covering that is egg-shaped and of a brownish-yellow color. It is about two millimeters in diameter. When the animal is quiet, no one would suspect its real nature, so much does it look like the seed of an aquatic plant. The testaceous envelope has a single round orifice at its more pointed end. The animal, when in an active state, pushes out the protoplasmic substance, which speedily gives off ramifying extensions, and these by further ramification and inosculation form a complicated network. The protoplasm of the animal also extends itself in such a way as to form a continuous layer on the external surface of the test. From this layer numerous protoplasmic threads may extend out, forming more or less complicated networks. By the alternate contraction and extension of its protoplasmic threads and networks, minute one-celled plants and animals are entrapped like flies in a spider’s web (d). When caught they are carried, by retraction of the protoplasmic thread-like pseudopodia, into the endoplasm in the test. Here the nutritious parts of the entrapped creatures are abstracted and assimilated. In transparent species, the indigestible parts, such as the silicious valves of diatoms, may be distinguished in the midst of the endoplasm, from which they are ultimately extruded.

When gromia oviformis reproduces by mitosis it gives off a bud (small cell), which finally separates from the parent form and constitutes a distinct individual. This process may be repeated many times, so that a great number of separate individuals may be formed, all of which lead detached and independent lives.

Most of the protozoa, which are produced by fission (cell division by mitosis), separate entirely from each other, as in Gromia; but in many of these unicellular animals, the new creatures produced by fission do not separate from one another, but remain more or less closely connected, and thus form colonies of Protozoans. These colonies are of the greatest interest, for they represent a lower stage of the cell colonies of the Metazoa (multicellular animals). They reproduce, in many cases, in a way which is strongly suggestive of reproduction in the Metazoa.

Microgromia socialis is a little unicellular animal, having a thin, nearly globular, calcareous shell that it secretes upon its surface. It multiplies by fission, and forms a number of distinct individuals which have the curious habit of fusing their pseudopodia and uniting into a more or less closely associated colony. The individuals sometimes remain at a distance from one another, but sometimes associate themselves together into a compact colony. These individuals are all alike, performing the same functions. There is no division of labor among the units, but they live practically an independent life. If the individual animals were detached from one another, they would live and build new colonies.

Codosiga umbellata is another unicellular animal—a flagellate Protozoan. It has a collar-like extension of its ectoplasm from the anterior extremity of its body, forming a sort of funnel from the bottom of which the thread-like structure (flagellum) arises. The vibrations of this flagellum cause a current of the surrounding fluid to set into the funnel so that particles of food reach the soft protoplasmic substance which serves as a mouth. The nucleus is seen near the base of the collar. Near the posterior extremity of the body two contractile vesicles are to be observed. This posterior extremity of the animal has a cylindrical extension of its ectoplasm by which it attaches itself to an object. This protozoan multiplies by longitudinal fission. In some species the animals separate completely from one another and lead entirely independent lives. But in Codosiga umbellata the fission does not extend through the cylindrical extension, so that a group of animals are associated in a colony.

Rotalia or Globigerina. The shelled amœba (Lagena) gives off a bud (unseparated cell) which grows to the full size, secretes its calcareous shell and remains connected with the parent form. This process may be repeated a number of times until a colony of shelled amœbæ, of varying pattern in different species, may be formed and permanently associated together ([Fig. 4]). The individual amœbæ are all alike and perform the same functions. There is no division of labor, no specialization, among them. If the individual animals could be separated from one another they would live and build new colonies.