D E F A, blood cell; B, ciliated epithelium; C, nerve cell; D, bone-forming cell; E, muscle cell; F, fibrous connective cell. other cells, a substance like jelly, called intercellular substance. This stands in the same relation to the cells as does mortar to the bricks in a wall. Several other types of cells might be mentioned, as blood cells, cartilage cells, bone cells, and nerve cells. A glance at the figure shows their great variety of shapes and sizes. Functions Common to all Animals. The same functions performed by a single cell are performed by a many-celled animal. But in the Metazoa the various functions of the single cell are taken up by the organs. In a complex organism, like man, the organs and the functions they perform may be briefly given as follows: (1) The organs of food taking: mouth and parts which place food in the mouth. (2) The organs of digestion: the food tube and the glands connected with it. The fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a fluid. Such fluid may then pass by osmosis through the walls of the food tube into the blood. (3) The organs of circulation: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. (4) The organs of respiration: the organs in which the blood. receives oxygen and gives up carbon dioxide. (5) The organs of excretion: such as the kidneys and skin, which pass off nitrogenous waste matter from the body. (6) The organs of locomotion: muscles and their attachments. and connectives, namely, tendons, ligaments, and bones. (7) The organs of nervous control: the central nervous system, which has control of coördinated movement. (8) The sense organs: such as sight, hearing, smell, taste, and touch. Almost all animals have the functions mentioned above. In most, the various organs mentioned are more or less developed, although in the simpler forms of animal life some of the organs mentioned above are either very poorly developed or entirely lacking. REFERENCE BOOKS FOR THE PUPIL Herrick, Text-book in General Zoology, Chap. III. American Book Company. Holder, Half Hours with the Lower Animals, Chap. I. American Book Company. French, Animal Activities, Chaps. III, XI. Longmans, Green, and Company. FOR THE TEACHER Dodge, General Zoology, Chaps. VII-X. American Book Company. body is pierced by a number of tiny holes or pores which communicate with the cloaca. The shape of the sponge is maintained by a skeleton composed of many tiny pieces or spicules of carbonate of lime. Notice the edge of the osculum. Make a drawing of the Grantia twice natural size, showing all the above structures.1 An examination with the microscope shows the pores of the sponge to be lined with ciliated cells. These, by means of movements of the cilia, set up a current of water toward the cloaca. This current bears food particles, tiny plants and animals, which are seized and digested by the ciliated cells. These cells seemingly pass on the food to the The middle layer 1 See Hunter and Valentine, Manual, page 159. Diagram of a simple sponge; i, inhalant opening; o, ex- other cells of the body. halant opening or oscu lum. of the body is composed of structureless material containing cells which secrete lime to form the spicules. Eggs and sperms are also developed in this layer and are set free when ripe to develop in the water. Spicules may be freed from the living part of the sponge by placing a Grantia in a strong solution of caustic soda for a few minutes. Mount a little of the sediment from the bottom of the dish in water or glycerine. Note the different forms of the spicules. Draw several for your notebook. Development of the Sponge. In the case of the sponge, as in most plants and animals, the life history begins with a single cell, the fertilized egg. This cell, as we remember, has been formed by the union of two other cells, a tiny (usually motile) cell, the sperm, and a large cell, the egg. After the egg is fertilized by a sperm cell, it splits into two, four, eight, and sixteen cells; as the number of cells increases, a hollow ball of cells called the blastula is formed (somewhat like the Volvox); later this ball sinks in on one side and a double-walled cup of cells, now called a gastrula, results. Stages in the segmentation of an egg, showing the formation of the gastrula. Practically all animals pass through the above stages in their development from the egg, although these stages are often not plain to see because of the presence of food material (yolk) in the egg. The gastrula, which swims by means of cilia, soon settles down, a skeleton is formed, other changes resulting in the formation of pores and osculum take place, and the sponge begins life as an adult. The early stages of life when an animal is unlike the adult are known as larval stages; the animal at this time is called a larva. The young sponge consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called ectoderm; the inner layer, developed from the inner layer of the gastrula, the endoderm. A middle structureless layer, called the mesoglea, is also found. In higher animals this layer (called mesoderm) gives rise to muscles and parts of other internal structures. (3) The horny fiber sponges. These, the sponges of commerce, have the skeleton composed of tough fibers of material somewhat like that of cow's horn. This fiber is elastic and has the power to absorb water. In a living state, the horny fiber sponge is a dark-colored fleshy mass, usually found attached to rocks. The warm waters of the Mediterranean Sea and the West Indies furnish most of our sponges. The sponges are pulled up from their resting place on the bottom, either by means of long-handled rakes operated by men in boats, or are secured by divers. They are then spread out on the shore in the sun, and the living tissues allowed to decay; then after treatment consisting of beating, trimming, and bleaching, the bath sponge is ready for the market. A horny fiber sponge; I.P., the incurrent pores; 0, osculum. Notice that this sponge is made up of apparently several individuals. One fourth natural size. HUNTER'S BIOL. - 13 Venus's flower basket: a sponge with a glassy skeleton. RELATION TO ENVIRONMENT. Sponges are found in both fresh and salt water.1 They are found 1 Only one species of fresh water sponge, Spongilla, is known in this country. |