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. [Illustration: 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.