[Illustration: Rockweed, a brown algae, showing its distribution on rocks below highwater mark.] The extent of the plant kingdom can only be hinted at; each year new species are added to the lists. There are about 110,000 species of flowering plants and nearly as many flowerless plants. The latter consist of over 3500 species of fernlike plants, some 16,500 species of mosses, over 5600 lichens (plants consisting of a partnership between algae and fungi), approximately 55,000 species of fungi, and about 16,000 species of algae. [Illustration: A moss plant. _G_, the moss body; _S_, the spore-bearing stalk (fruiting body).] Development of a Simple Animal.--Many-celled animals are formed in much the same way as are many-celled seed plants. A common bath sponge, an earthworm, a fish, or a dog,--each and all of them begin life in the same manner. In a many-celled animal the life history begins with a single cell, the fertilized egg. As in the flowering plant, this cell 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; later this ball sinks in on one side, and a double-walled cup of cells, now called a _gastrula_, results. 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. In animals the body consists of three layers of cells: those of the outside, developed from the outer layer of the gastrula, are called _ectoderm_, which later gives rise to the skin, nervous system, etc.; an inner layer, developed from the inner layer of the gastrula, the _endoderm_, which forms the lining of the digestive organs, etc.; a middle layer, called the _mesoderm_, lying between the ectoderm and the endoderm, is also found. In higher animals this layer gives rise to muscles, the skeleton, and parts of other internal structures. [Illustration: Stages in the development of a fertilized egg into the gastrula stage. Read your text, then draw these stages and name each stage.] Physiological Division of Labor.--If we compare the amoeba and the paramoecium, we find the latter a more complex organism than the former. An amoeba may take in food through any part of the body; the paramoecium has a definite gullet; the amoeba may use any part of the body for locomotion; the paramoecium has definite parts of the cell, the cilia, fitted for this work. Since the structure of the paramoecium is more complex, we say that it is a "higher" animal. In the vorticella, a still more complex cell, part of the cell has grown out like a stalk, has become contractile, and acts like muscle.