stance. The bodies of all jelly-fishes are soft and gelatinous, the body substance containing hardly one per cent of solid matter. It is mostly water. Many jelly-fishes are beautifully and strikingly colored, and as they swim slowly about near the surface of the ocean, lazily opening and shutting their iridescent, umbrella-like bodies, they are among the most beautiful of marine organisms. When one of the jelly-fishes is taken from the water, however, it quickly loses its brilliant colors, and dries away to a shapeless, shriveling, sticky mass. Some of the most beautiful of the jelly-fishes belong to a group called the Siphonophora. These jelly-fishes are elongate and tube-like rather than umbrella- or bell-shaped, and they are polymorphic—that is, there are several different forms of individuals belonging to a single kind or species. The Siphonophora are all free-swimming, but nevertheless form small colonies. In the Mediterranean Sea and in other southern ocean waters the surface may be covered for great areas by these brilliantly colored jelly-fish colonies, each of which looks, as a celebrated German naturalist has said, like a swimming flower cluster whose parts, flowers, stems, and leaves seem to be made of transparent crystal, but which possess the life and soul of an animal. An abundant species of these Siphonophora (Fig. 25) is composed of a slender, flexible, floating, central stem several feet long, to which are attached thousands of medusa and polyp individuals representing several different kinds of forms, each kind of individual being specially modified or adapted to perform some one duty. The central stem is a greatly elongated polyp individual, whose upper end is dilated and filled with air to form a float. This individual holds up the whole colony. Grouped around this central stem just below the float are many bell-shaped bodies which alternately open and close, and by thus drawing in and expelling water from their cavities impel the whole colony through the water. These bell-shaped structures are attached me dusa individuals, whose business it is to be the locomotive organs for the colony. These medusæ are without tentacles, and take no food and produce no young. They have given up the power of performing these other life processes, and devote themselves wholly to the business of locomotion. From the lower end of the central stem rises a host of structures, among which several distinct kinds are readily perceived. One kind is composed of a pearshaped hollow body open at its free end, and bearing a long tentacle which is furnished with numerous groups of stinging cells. These are the polyp individuals whose especial business it is to capture and sting prey and to eat it. These individuals are the food-getters for the colony. Scattered among these stinging, feeding polyps, are numerous smaller individuals with oval, closed body, each bearing a long, slender thread. These threads are very sensitive, and the polyps bearing them have for special function that of feeling or being sensible of stimuli from without. They are the sense organs or sense individuals of the colony. Finally, there are two other kinds of structures or individuals which produce the special reproductive cells for the perpetuation of the species. These are the modified medusa individuals, and one kind, larger than the other, produces the active sperm cells, while the other produces the inactive egg cells. 27. Increase in the degree of complexity.-In the corals, sea-anemones, and jelly-fishes there is plainly much more of a division of labor among the various parts of an individual and much more modification of these parts-that is, much more structural complexity than among the sponges and Hydra. And these, in their turn, are more complex than are the colonial Protozoa, the Volvocinæ. There is a great difference in degree of complexity among the slightly complex animals. But the various groups of these animals which we have studied can all be arranged roughly in a series beginning with the least complex among them and ascending to the most complex. And in this series, and in the always accompanying division of labor among the different parts, the gradual increase in complexity is beautifully shown. From an animal composed of many structurally similar cells, each cell capable of performing all the life processes, we pass to an animal composed of cells of a few different kinds, of slight structural diversity. Each kind of cell devotes itself especially to a certain few life processes or functions. Next we find an animal in which the cells of one kind are specially aggregated to form a single part of the body which is specially devoted to the performance of a single function. This diversity among the cells increases, this aggregation of similar cells to form special parts or organs increases, and the division of labor or assignment of special functions to special organs becomes more and more pronounced. Among the more complex polyps and jelly-fishes the contractile cells form distinct muscle fibers and muscles; the sensitive cells form distinct nerve cells and nerve fibers which are arranged in a primitive nervous system; the digestive cavity becomes complex and composed of different portions; the reproductive cells are formed by special organs, and the distinction between the egg cells and the sperm cells—that is, between the female reproductive elements and the male reproductive elements-becomes more pronounced. We have followed this increase or development of structural and physiological complexity from simplest animals to fairly complex ones. The principle of this development of complexity is evident. It will not be profitable to attempt to follow in detail this development among the higher animals. The complex animals are complex because their life processes are performed by special parts of their body, which parts are specially modified so as to perform these processes well. The animals which are more complex than those we have studied differ from these simply in the degree of complexity attained. In order to understand this better we shall not further consider special groups of animals, but special processes or functions, and attempt to see how the modification and increase in complexity of structure goes hand in hand with the increase of elaborateness or complexity in the performance of function. CHAPTER III THE MULTIPLICATION OF ANIMALS AND SEX 28. All life from life.-On the performance of the function of reproduction or multiplication depends the existence or perpetuation of the species. Although an animal may take food and perform all the functions necessary to its own life, it does not fulfill the demands of successful existence unless it reproduces itself. Some individuals of every species must produce offspring or the species becomes extinct. We have seen in our study of the simple animals that the function of reproduction is the first function to become differentiated in the ascent from simplest animals to complex animals. The first division of labor among the cells composing the bodies of the slightly complex animals and the first structural differences among the cells are connected with the performance of the function of reproduction or multiplication. We are all so familiar with the fact that a kitten comes into the world only through being born, as the offspring of parents of its kind, that we shall likely not appreciate at first the full significance of the statement that all life comes from life; that all organisms are produced by other organisms. Nor shall we at first appreciate the importance of the statement. This is a generalization of modern times. It has always been easy to see that cats and horses and chickens and the other animals we familiarly know give birth to young or new animals of their own kind; or, put conversely, that young or new cats and horses and chickens come into existence only as the off |