As we have seen in plants, this results in a large number of collections of cells in the body, each collection alike in structure and performing the same function. Such a collection of cells we call a tissue. (See Chapter III.)

Frequently several tissues have certain functions to perform in conjunction with one another. The arm of the human body performs movement. To do this, several tissues, as muscles, nerves, and bones, must act together. A collection of tissues performing certain work is called an organ.

In the sponge, division of labor occurs between the cells of the simple animal, some cells lining the incurrent pores creating a current of water, and feeding upon the minute organisms which come within reach, other cells building the skeleton of the sponge, still others becoming eggs or sperms. Division of labor of a more complicated sort is seen in the hydra. Here the cells which do the same kind of work are collected into tissues, each tissue being a collection of cells, all of which are more or less alike and do the same kind of work. But in higher animals which are more complicated in structure and in which the tissues are found working together to form organs, division of labor is still more developed. In the human arm, an organ fitted for certain movements, think of the number of tissues and the complicated actions which are possible. The most extreme division of labor is seen in the organism which has the most complex actions to perform and whose organs are fitted for such work.

In our daily life in a town or city we see division of labor between individuals. Such division of labor may occur among other animals, as, for example, bees or ants. But it is seen at its highest in a great city or in a large business or industry. In the stockyards of Chicago, division of labor has resulted in certain men performing but a single movement during their entire day's work, but this movement repeated so many times in a day has resulted in wonderful accuracy and increased speed. Thus division of labor obtains its end.

Tissues in the Human Body. - Every animal body above the protozoan is composed of a certain number of tissues. The cells making up these tissues have certain well-defined characteristics. In very simple animals the cells are all very much alike, but in more

complex animals the cells are more and more unlike as their work becomes more and more different. Let us see what these cells may be, what their structure is, and, in a general way, what function each has in the human body.

Muscle Cells. A large part of our body is made up of muscle. Muscle cells are elongated in shape, and have great contractile power. Their work is that of causing movement, and this is usually done by means of attachment to a skeleton inside the body. In man they may be of two kinds, voluntary (under control of the will) and involuntary.

m

n

Diagrams of sections of cells, greatly magnified. e, flat cell (epithelium) from mouth; c, columnar epithelium from food tube; b, bone-forming cell; l, liver cell; m, muscle cell; f, fat cell; n, nerve cell.

Epithelial Cells. Such cells cover the outside of a body or line the inside of the cavities in the body. The shape of such cells varies from flat plates to little cubes or columns depending upon their position inside or outside the body. Some bear cilia, an adaptation. Can you think of their purpose?

Connective Tissue Cells. Such cells form the connection between tissues in the body. They are characterized by possessing numerous long processes. They also secrete, as do many 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 general 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: food may be taken in by individual cells, as those lining the pores of the sponge, or definite parts of a food tube may be set apart for this purpose, as the mouth and parts which place food in the mouth.

(2) The organs of digestion: the food tube and collections of cells which form the glands connected with it. The enzymes in 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. In simple forms of Metazoa, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis.

(4) The organs of respiration: the organs in which the blood receives oxygen and gives up carbon dioxide. The outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals.

(5) The organs of excretion: such as the kidneys and skin, which pass off nitrogenous and other waste matters 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. This consists of scattered cells in low forms of life; such cells are collected into groups and connected with each other in higher animals.

(8) The sense organs: collections of cells having to do with the reception of sight, hearing, smell, taste, and touch.

(9) The organs of reproduction: the sperm and egg-forming glands.

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.

Sponges may be placed, according to the kind of skeleton they possess, in the following groups:

(1) The limy sponges, in which the skeleton is composed of spicules of carbonate of lime. Grantia is an example.

Here the skeleton Some of the rarest sponges belong in

(2) The glassy sponges. is made of silica or glass. and most beautiful of all this class. The Venus's flower basket is an example.

(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 darkcolored fleshy mass, usually found attached to rocks. The warm waters of the Mediterranean Sea and the West Indies furnish most

a

Venus's flower basket; sponge with a glassy skeleton.

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, bleaching, and trimming, the bath sponge is ready for the mar

Medusa (Gonionemus murbachii), showing tentacles, mouth, digestive canals, and reproductive bodies. Photographed from the model at the American Museum of Natural History.

ket.

CELENTERATES

The hydra and its saltwater allies, the jellyfish, hydroids, and corals, belong to a group of animals known as the Cœlenterata. The word "coelenterate" (calom-body cavity, enteron-food tube) explains the structure of the group. They are animals which have a common body cavity and food tube, the animal in its simplest form being little more than a bag.

Medusa.

Among the most interesting of all the cœlenterates inhabiting the salt water are the jellyfishes or medusæ. These animals vary greatly in size from a tiny umbrella-shaped animal little larger than the head of a pin to huge jellyfish several feet in diameter.

Development. Many species of medusa pass through another stage of life. As medusæ they reproduce by eggs and sperms, that is, sexually. The egg of the medusa segments, forming ultimately a ball of cells (the blastrula) which swims around by means of cilia. Ultimately the little animal settles down on one end and becomes fixed to a rock, seaweed, or pile. The free end becomes indented in the same manner as a hollow rubber ball may be pushed in on one side. This indented side becomes a mouth, tentacles develop around the orifice, and we have an animal that looks very much like the hydra. This animal, now known as a hydroid polyp, buds rapidly and soon forms a colony of little polyps, each of which is connected with its neighbor by a hollow food tube. The hydroid polyp differs from its fresh-water cousin, the hydra, by usually possessing a tough covering which is not alive.

Alternation of Generations in Colenterates. The lives of a hydroid and a medusa are seen thus to be intimately connected with each other. A hydroid colony produces new polyps by budding. This we know is an asexual method of reproduction. There come from this hydroid colony, however, little buds which give rise to medusæ. These medusæ produce eggs and sperms. Their reproduction is sexual, as was the reproduction by means of eggs and sperms from the prothallus of the fern. So we have in animals, as well as in plants, an alternation of generations.

Sea anemone. About one half natural size. The right-hand specimen is expanded. Note the mouth surrounded by the tentacles. The left-hand specimen is contracted. From model at the American Museum of Natural History.