to assign such forms to a definite position among the sponges.

36. Structure of a simple sponge.-In the simpler sponges the body is usually vase-shaped (Fig. 16), with the base

fastened to some foreign. object, while at an opposite end an opening leads into a comparatively large internal cavity. This latter space is also put in communication with the exterior by a multitude of minute pores which penetrate the body wall.

FIG. 15.-Various forms of sponges, natural size. (From Nature.)

the living condition currents of water continually pass through these smaller canals, and out of the large terminal opening, thus bringing within reach of the body minute

floating organisms or organic remains which serve as food. The mechanism by which this process is effected, and the various other structures of the body, are in large part invisible from the exterior, requiring the

study of thin sections of the sponge to make them clearly understood.

Under the microscope such a section shows the body of a sponge to consist of an immense number of variously formed cells constituting three distinct layers (Fig. 17). Not only do these layers consist of different kinds of cells, but the duties performed by each are different. For example, a glance at Fig. 17 will show that in the inner layer certain columnar cells exist, provided with a flagellum and encircling collar, the appearance being strikingly like certain of the Protozoa (Fig. 8, B). During life their whip-like processes, lashing backward and forward in perfect unison, produce currents of water which continually pass through the body. The food thus entering the animal is taken up by the cells of the inner layer as it passes by. The supply, however, is usually more than sufficient to meet the demands of this layer, and the excess is passed on to the middle and outer layers. The exact method by which this occurs is still a matter of doubt, but there seems to be little question but that each cell of the body receives its food in a practically unmodified condition, requiring that it digest as well as assimilate. The oxygen necessary to this latter process

FIG. 16.-One of the simplest sponges (Calcolynthus primigenius (after HAECKEL). A portion

of the wall has been removed to show the inside.

is absorbed by all parts of the body in contact with the water.

37. Skeleton of sponges.-When it is remembered that the protoplasm composing the cells of the sponge has about

m

FIG. 17.-Portion of wall of sponge, showing three layers. e, outer layer; i, inner layer, consisting

of collared cells; m, middle layer, consisting of irregular cells, among which are the radiate spicules and egg-cells.

the same consistence as the white of egg, it will be readily understood why the greater number of sponges possess a skeleton. Without such a support the larger globular or branched forms could not exist, and even in the

smaller members there would be danger of a collapse of the body walls and consequent stoppage of the food supply, owing to the closure of the pores. So in all but a very few thin or flat forms a skeleton appears in the young sponge almost before growth

has fairly begun, and this increases with the body in size and complexity. It is formed by the activity of the cells of the middle layer, and may be composed either of a lime compound resembling marble, or of flint, or of a horn-like substance resembling silk, or these may exist in combination in certain species. When consisting of either of the first-named substances it is never formed in one continuous piece, but of a vast multitude of variously shaped crystal-like bodies termed spicules (Fig. 18). These occur everywhere throughout the body, firmly bound together

FIG. 18.-Different types of sponge spicules.

by means of cells, or so interlocked that they form a rigid support to which the fleshy substance is bound and through which the numerous canals penetrate.

In a relatively few species only does the skeleton consist of horn, though there are many in which horn and flint exist together. In the former event, if the skeleton be elastic and of sufficient size, it becomes valuable to others than the naturalist, for the familiar sponges of commerce are the horny skeletons of forms usually taken in the West Indies or in the Mediterranean Sea. In these localities the animals are pulled off by divers, or with hooks, and are then spread out in shallow water where the protoplasmic substance rapidly decays. The remaining skeleton, thoroughly washed and dried, is ready for the markets of the civilized world.

Examining a bit of such a "sponge" under a magnifying glass, it will be seen that the skeleton is not composed of various pieces, but of one continuous mass of branching fibers, which interlace and unite in apparently the greatest confusion; yet in the living animal these were perfectly adapted to the position of the canals and the general needs of the animal.

Besides being a scaffold-work to which the fleshy portions of the body are fastened, the skeleton serves also for protection. In some species, needle-like spicules as fast as they are formed are partly pushed out over the entire surface of the body, giving the appearance of a spiny cactus; or in other cases they are arranged in tufts about the canals, effectually preventing the entrance of any marauder. Thus perfectly protected, the sponges have but few natural enemies, and hence it is that in favorable localities they grow in great profusion.

38. Race histories and life histories.-We have now traced living things from their simplest beginnings, where they exist as single cells, and have seen that in bygone times similar forms have united into simple colonies, and these

through a division of labor among the constituent cells have resulted in Volvox-like colonies. There are the strongest reasons for the belief that as these simple forms scattered into various surroundings and underwent changes to meet the shifting conditions, they assumed different degrees of complexity that have resulted in the animal forms of the present day.

It may have been noticed also that the sponge in its development passes through these stages: a single-celled egg; later, a young form similar to Pandorina, then growing to look like Volvox, and finally assuming its permanent form. The history of the race of sponges and their development through a long line of ancestry of increasing complexity is thus told by the sponge as it develops from the egg into the adult; and, so far as we know, all the many-celled animals in their growth from the egg repeat more or less clearly the stages passed through by their forefathers.