[Illustration: 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. [Illustration: FIG. 42.--Chænonotus, a lower worm. _m_, Mouth; _e_, eye; _ss_, sensory hairs; _œ_, œsophagus; _sk_, skin; _d_, digestive canal; _n_, nephridia; _ex_, excretory opening; _c_, cilia; _a_, anus; _b_, brain; _mc_, muscle cells; _n_, nerves; _o_, ovary.] Thirdly, we must briefly allude to the Nemertines. These are a group of flattened thread-like worms of very variable size, found both in fresh and salt water. The most notable advance in this group is to be seen in the occurrence of a special circulatory system. It has already been indicated that the gastric cavity of the lower forms has the double function of digestion and of the transport of nutritive substances to the various parts of the body. In the Nemertines the second of these functions is carried out by the blood system, which consists of two or three vessels that run parallel throughout the length of the body and anastomose at either end. There is no indication of any enlarged or specially contractile portion of any of these, no indication, that is to say, of a heart. The blood conveys not only nutritive substances, but also, as in the higher animals, oxygen. Some Nemertines have indeed red blood, containing true hæmoglobin, which is well known as the oxygen-carrying material in the vertebrates. A typical Nemertine is shown in Fig. 44, and a diagram showing some features of the anatomy in Fig. 45. It will be seen that the nervous system is of the same type as in the worms already described. There are two pairs of sense organs, one pair being eyes, and the other probably having the function of gauging the chemical nature of the water. The Nemertines possess a peculiar organ in a snout or proboscis, which they can protrude or withdraw into a special sac. The snout is armed with a sharp sting, and