CHAPTER IV

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

FIG. 12.-Pandorina (from Nature).
Highly magnified.

Sequence. As a result of h a union they have not their independence, for if separated from the main company it continues to exist. From such a simple colonial type we may pass through a series of several more complex forms which reach their highest development in the beauful organism, Volvox (Fig. 13). In this form the indidual members, to the number of many thousand, are arged in the shape of a hollow sphere. The united efforts the greater number, which bear on their outer surfaces o flagella, drive the colony with the rolling movement

same functions are performed by definite structures, the cilia and gullet. Now, it is well known that a workman is able to make better watch-springs, when this is his sole duty, than another who must make all parts of the watch; and likewise, where a definite task is performed by a definite structure, it is more efficiently done than where any and every part of the body must carry it on. So the Amaba, in which definite tasks are performed by any part of the body indifferently, is less perfect and thus lower than the Paramecium, where these functions are performed by special organs. As we ascend the scale of life we find this division of labor among special parts of the body more complete, the organs, and therefore the animal, more complex, and better fitted to carry on the work of its life.

CHAPTER IV

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 be separated from the main company it continues to exist.

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 individual 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 the power of division, and the egg enters upon its development, dividing again and again. The cells so pro

duced 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 examination 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 into 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.