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THE SLIGHTLY COMPLEX ANIMALS OR SPONGES
32. Their relation to the Protozoa.—While the greater number of one-celled forms are not united with their fellows, there are several species where the reverse is true. In Fig. 12, for example, a fresh-water form known as Pandorina is represented, consisting of sixteen cells embedded in a spherical, jelly-like substance, each one of which is precisely like its companions in form and activity. The aggregation may be looked upon as a colony of sixteen Protozoa united together to derive the benefit of increased locomotion and a larger amount of food in consequence. As a result of such a union they have not lost their independence, for if one pe separated rom the main one be separated from the main
Fig. 12.- Pandorina (from Nature). company it continues to exist. Highly magnified.
From such a simple colonial type we may pass through a series of several more complex forms which reach their highest development in the beautiful organism, Volvox (Fig. 13). In this form the indi. vidual members, to the number of many thousand, are arranged in the shape of a hollow sphere. The united efforts of the greater number, which bear on their outer surfaces two flagella, drive the colony with the rolling movement
from place to place. As just indicated, some individuals lack the flagella, and their subsequent careers show them to be of a peculiar type. Sooner or later each undergoes
a series of divisions forming a little globe of cells, which migrates into the interior of the parent sphere and develops into a new colony. Within a short time the walls of the parent break, liberating the imprisoned young, which continue the existence of the species while the parent organism soon decays.
Under certain circumstances, instead of developing colonies by such a method, some of the cells may store up food matters and become eggs, while others, known as sperm-cells, develop a flagellum, and separating from the colony swim actively in the surrounding water, where each finally unites with an egg. This union, like that of the two individuals in Vorticel
la (Fig. 10, b, c), results in Fig. 13.-A, Volvox minor, entire colony the power of division, and
(from Nature). B, C, and D, reproduc- the egg enters upon its detive cells of Volvox globator. All highly velopment, dividing again magnified.
and again. The cells so produced remain together, form a sphere, and finally develop a Volvox colony.
In such associations as Volvox an important step has been taken beyond that of Pandorina, for there is a division of the labors of the colony among its various members, some acting as locomotor cells while others are germ-cells. These are now so dependent one upon the other that they are unable to exist after separation from the main company, just as a part of the squirrel is incapable of leading an independent existence. A higher type of organism has thus arisen intermediate between the simple one-celled animals and those of many cells, especially the sponges—a relation which is more readily recognized after an examina. . tion of the latter.
33. Development of the sponge.—Like all many-celled animals, the sponge begins its life, however, as a single cell
-the egg—which is in this case barely visible to the sharp unaided eye. Fertilized by its union with a sperm cell, development commences, and the first apparent indication of the process will be the division of the cell into halves (Fig. 14, A, B). Each half redivides înto four, these again into eight cells, and this process is repeated, giving the young sponge the general form of Pandorina. The divisions of the cells still continue and result in the formation of a hollow globe of cells (called the blastula, Fig. 14, E, F) similar to Volvox, and at this point the young larva leaves the parent.
The next transformation consists in a pushing in of one side of the sphere, just as one might press in the side of a hollow rubber ball. The depression gradually deepens, and finally results in the formation of a two-layered sac known as the gastrula (Fig. 14, G). At this stage of its existence the sponge settles down for life in some suitable spot, by applying the opening of its sac-like body to some foreign object. In assuming the final form a new mouth breaks through what was once the bottom of the sac, canals perforate the body wall, a skeleton is developed, and the characteristic features of the adult are thus attained.
34. Distribution.—The sponges are aquatic animals, and, with the exception of one family consisting of relatively few species, all are inhabitants of the sea in every part of
Fig. 14.- Diagrams illustrating the development of a sponge. A, egg-cell ; n, nucleus.
B, C, D, 2., 4., and 16-cell stages. E, blastula. F, section through somewhat older larva. G, gastrula. H, young sponge. I, section through somewhat younger larva than H.
the globe. The larger number occupy positions along the shore, becoming especially abundant in the tropics; but other species occur at greater depths, several species living
between three and four miles from the surface. Unlike the majority of animals, all members of this group are securely fastened to some foreign object, such as rocks, the supports of wharves, or with one extremity embedded in the sand. As we have seen, the young enjoy a free-swimming existence and are swept far and wide by means of tidal currents, but sooner or later these migrations are terminated in some suitable locality, where the sponge passes the remainder of its existence. During this time some species may never exceed the size of a mustard-seed, while others attain a diameter of three feet, or even more. Sponges also vary exceedingly in shape, some having the form of thin encrusting sheets, others being globular, tubular, cuplike, or highly branched (Fig. 15). V 35. The influence of their surroundings.— In by far the larger number of cases an animal possesses the bodily form of the parent. External agencies may modify this to some extent, but usually only to a limited degree. A squirrel, for example, resembling its parent, may grow to a relatively large or stunted size according to the food supply, and it may become strong or weak according to the amount of exercise, and various other changes may result owing to outside causes; but as a result of these influences the animal is rarely so modified that one is unable to distinguish the species. Many of the sponges, however, are exceptions to this general rule. If, for example, some of the young of a certain parent develop in quiet water or in an unfavorable locality, they will usually be low, flat, and unbranched; while the others, growing in swiftly running, waterways, develop into tall, comparatively delicate and highly branched individuals. Under such circumstances not only does the external form become modified, but the internal organization may undergo profound change. The entire organism is plastic and readily molded by the influence of its surroundings, and the consequent lack of definite characters often renders it impossible