The fundamental importance of this type is that we have already a division of the life activities. The majority of the cells are concerned in the nutrition of the individual as a whole. These ultimately perish. A minority, however, are fed and protected by them, and these in return secure the perpetuation of the race. This division into a mortal 'body' portion and an immortal reproductive portion is the first and most important division of the life activities, whether in the animal or in the plant kingdoms. The body cells, modified in various directions for their special purposes, could not, and do not, reproduce complete new individuals. Therefore a generalised type of cell is maintained for the express purpose of the propagation of the race. It is to be observed, now, that the process of reproduction in Volvox is not always such as we have described. Sometimes the reproductive cells are of two kinds. The one type divides into a great number of small ciliated cells, which escape separately and directly to the outside of the sphere, and swim away. These free-swimming individuals do not form new colonies, but seek out the reproductive cells of the other type, which latter still form part of the organism which has produced them. One of the free-swimming cells enters each of those of the other kind, and the nuclei of the two merge into one. The cell so produced, after a longer or shorter rest period, commences to divide and redivide in the manner already described, forming a new colony. The process that we have described is that of sexual reproduction, and its essential features are the same as in Volvox throughout the whole animal kingdom. The small free-swimming cells are the male reproductive bodies or sperms, the others are the female or egg cells. The union of the two produces the fertilised egg, and the process of union is termed fertilisation. In Volvox, the male and female elements are sometimes produced by the same individual, at other times by different ones. Separation of the sexes is no necessary accompaniment of the process of sexual reproduction, and indeed it is only in the higher groups of animals that separate sexes are the rule. The various conditions in Volvox are illustrated in Figs. 28, 29, and 30.
Fig. 30.—Volvox. Portion of a hermaphrodite individual, showing egg cells (O, O1), and sperms (S1 S2 S3).
The next great groups of animals are, on the one hand, that of the sponges, and, on the other, that which includes the sea-anemones, jelly-fishes, corals, etc. At first sight their structure seems vastly different to that of the Volvox, from some form similar to which they have probably been derived. The evidence obtained from the study of their individual development, however, strongly suggests a process by which we suppose that they evolved from Volvox-like ancestors. We shall therefore briefly describe the earlier stages of the development of a coral. The sexually produced individual starts life as a single cell, the fertilised egg. This divides and redivides until a hollow ball of cells is produced, which cells, like those of the Volvox, bear cilia. Although simply spherical in shape, the creature moves by rotating round a definite axis, like a planet. Moreover, nutriment is absorbed not by any or every part of the surface, but only by a small area round the lower pole. Now as development proceeds, the cells at this pole divide more rapidly than the rest, with the natural result that the ball begins to get out of shape. The distended portion, however, develops to the inside, so that one part of the sphere is, as it were, pushed into the other. When this process has been completed, the original internal cavity is almost entirely eliminated, and a form is produced which resembles a double-walled flask or vase. Such a form may be taken as the fundamental architectural type of the groups that we are now to consider. The meaning of this further step of Evolution is again specialisation. The inner layer of cells takes on the functions of digestion and absorption of food, there having been evolved, in fact, the simplest possible form of mouth and stomach. Such other functions as those of locomotion, protection, and support are exercised by the outer layer. This process is illustrated in Fig. 31.
Fig. 31.—Process of gastrulation in a coral.
A, B, Blastula, or simple hollow ball; C, D, intermediate condition; E, F, gastrula, or double-walled flask condition.
But there is no known type of animal which, in its adult form, shows quite the simple structure that we have described. Perhaps the nearest approach is to be found in the lower sponges, in which two modifications of the original plan have already been introduced. In the first place, the creature is sedentary, being fixed, in an inverted position, to some solid basis. It has, so to speak, ceased to be a hunter, and is become a fisher. Secondly, its wall is pierced in many places, so as to permit of a freer circulation, through the digestive cavity, of the water which contains the food material. The water passes in through these numerous perforations, and out through the main central opening or 'mouth.' The sponges do not appear to represent a stage in the main line of Evolution, but lead us almost immediately into a cul-de-sac. We therefore cannot pause to describe fully the many peculiar and interesting developments which occurred in the group. An ordinary 'sponge,' by the way, bears the same relation to the creature which produces it as does a 'coral' to the coral animal. It represents, that is to say, the skeletons of a large colony of individuals. The structure of a sponge is shown in Fig. 32.
Fig. 32.—Diagrammatic section of lower sponge.