The worms comprise many greatly divergent groups, and the difference between the lowest and the highest of these has been produced by many important steps in Evolution. Of these groups but few immediately concern us; the first and lowest of those which do, is that of the Turbellarians, a section of the Platodes or flat-worms. The Turbellarians are small or microscopic tongue-shaped organisms, of which the majority of species live on the sea-floor, others however being found in fresh water. The surface of the body is covered uniformly with cilia, which serve, in the smaller forms, as organs of propulsion, while in the larger they appear to have the function of maintaining a flow of fresh water over the surface, and thus of assisting respiration. In some respects there has been little advance from the condition of the Cœlenterate. The digestive cavity is a simple or more or less divided sac, communicating with the exterior only by means of the mouth. Unlike the condition of affairs in the Cœlenterates and Ctenophora, however, the sex glands do not discharge the reproductive bodies into the digestive cavity, but directly to the exterior by means of a special opening. Each individual has a pair of male and a pair of female reproductive glands, but the eggs are not self-fertilised; nor is the fertilisation of the eggs trusted to chance and the sea-water, as in the lower groups. Instead there is a definite exchange of sperms between two individuals, and the eggs are fertilised before they leave the body. They are also frequently supplied with a store of nutritive material by a pair of special yolk glands. A distinct step of progress can thus be recognised in the arrangements for reproduction. Between the outer skin and the inner digestive layer is developed a considerable mass of cells, forming muscular and connective tissue, etc. It will readily be understood that the development of such thick tissue masses occasions two distinct new difficulties in the animal economy; for where cells are in direct contact neither with the digestive layer nor with the exterior, their nutrition and the removal of their waste products can no longer be efficiently carried on without special devices. Hence on the one hand a circulatory system, for the transport of food materials, and on the other an excretory system, become necessary. The first of these new departures was not destined to be made until the next stage of progress; the Turbellarians seem to have temporarily got over the difficulty, like the Ctenophora, by developing a complex and ramifying digestive cavity. An excretory system, however, makes its appearance here. Indeed, the beginnings of such a system can be seen in the Ctenophora, in which there are small excretory organs opening into the digestive cavity. The corresponding organs in the worms, as in all subsequent types, open directly to the outside. In the Turbellarians these organs, which are termed nephridia, are two in number, and consist of long tubes which branch and ramify throughout the body, the small branches terminating in special excreting cells, and the whole constituting a complete and thorough drainage system. The nervous system consists of one or two small masses of nerve cells termed ganglia in the front region, with a somewhat complex network of nerves connecting them with various parts of the body. There are frequently two pairs of sense organs, probably rudimentary eyes and ears respectively. The main features of the digestive, reproductive, excretory, and nervous system are shown in the figures in Fig. 40.
Fig. 40.—A simple Turbellarian—Rhabdocœlum (diagrammatic).
m, Mouth; d, digestive cavity; nc, nephridia; au, eyes; na, sense organs; g, brain; n, nerves; h, male, and e, female, reproductive glands.
The Turbellarians probably arose from Ctenophora or from some nearly related form, a view that receives support from the occurrence of several apparently intermediate types. The differences may, in fact, be partly accounted for as adaptations to meet the change of habitat from that of the upper waters to that of the sea floor. A spherical, or pear, or bell shape is suitable enough for a swimming animal, but would be impossible for one that was to crawl. The first change, then, we may imagine, was a flattening, which produced a disc-shaped animal, with the mouth in the centre of the lower aspect and the sense organs in the middle of the upper. Secondly, a definite mode of progression, by which one part of the body continually went first, would be an advantage, as permitting of a better co-ordination of movements, and an elongation of the body in the line of movement would have the effect of diminishing resistance and of making progression easier. Finally the sense organs, like the scouts of an army, would be best in front, and would migrate thither, and the mouth, in order to get the full benefit of the food which the sense organs sought out, would gradually shift to a position beside them. These adaptions, it is obvious, have produced a complete change in the architecture of the animal. Our sea-anemone, or Medusa, or Ctenophore is radially symmetrical. That is to say, its parts are arranged like the spokes of a wheel, and it may be divided into two equal halves by each of several planes passing through the main axis. It has an upper and a lower surface, but no head and tail ends. The lowest of the worms now can be divided into two halves only in one direction, that which separates the right and left sides. They are, in scientific language, bilaterally symmetrical. The change to this type of architecture was a very important step of Evolution, particularly in relation to locomotion. Bilateral symmetry was destined to remain a constant feature of three of the four great groups that evolved from the worms. The star-fishes reverted to the earlier condition.
Fig. 41.—A primitive flat-worm—Aphanostomum (× 50).
a, Mouth; g, sense organ; i, internal digestive tissue; s, male, and o, female, reproductive glands; with m and f, external openings.
The next class of worms with which we have to deal is that of the Rotifera. In their general structure, and in their excretory and sensory-nervous systems, the Rotifers do not differ essentially from the Turbellarians. They do differ, however, in that the digestive cavity has a second opening to the exterior, at the end opposite to the mouth. The advantage of this arrangement, which was retained in the subsequent stages of Evolution, is obvious, for it renders possible a much more regular and thorough digestive process. Instead of the food passing in, and the undigested remains passing out, by the same opening, and instead of the contents of the digestive cavity being a general mixture of food material in all stages of digestion, there is now a regular stream of food passing through the cavity in one direction, and being digested as it goes. A near relative of the Rotifers is shown in Fig. 42.
