336. But in some tribes even of these simple creatures there is visible by the microscope an afflux of their nutritive juices to the delicate pellicle that envelops them, in the form of a vascular net-work, in which there appears to be a motion of fluids, probably the nutritive juices flowing in the only position of the body in which they could come into direct contact with the surrounding element. In some more highly advanced tribes, as in wheel animalcules, there is an obvious circulating system in vessels near the surface of the skin. In other tribes, the internal surface constituting the alimentary canal, is of great extent and width, and forms numerous cavities which are often distended with water. In this manner a portion of the internal, as well as the external surface is made contributary to the function of respiration, and this extended respiration is conducive to their great and continued activity, to their rapid development, and to the extraordinary fertility of their races. [Illustration: Fig. CXXV.—_Medusa._ 1. The mouth; 2. the stomach; 3. large canals going from the stomach; 4. smaller canals which form; 5. a plexus of vessels at the margin of the disc serving for respiration; 6. margin of the disc.] 337. In creatures somewhat higher in the scale, a portion of the external surface is reflected inwards in the form of a sac, with an external opening (fig. CXXV. 1). In some medusæ there are numerous sacs of this kind, which pass inwards until they are separated only by thin septa from the cavities of the stomach. The water permeating and filling these sacs comes into contact with an interior portion of the body, not to be reached through the external surface. At the margin of the disk (fig. CXXV. 6) there is spread out a delicate net-work of vessels (fig. CXXV. 5); these vessels communicate with small canals (fig. CXXV. 4) which open into larger canals (fig. CXXV. 3) that proceed directly from the stomach (fig. CXXV. 2). As the aliment is prepared by the stomach, it is transmitted thence by these communicating canals to the exterior net-work of vessels where it is aërated. 338. As organization advances, as the component tissues of the body become more dense, and are moulded into more complex structures, when, moreover, these structures are placed deep in the interior of the body, far from the external envelop, and proportionally distant from the surrounding element, the respiratory apparatus necessarily increases in complexity. The first complication consists in the formation of minute, delicate, transparent tubes (fig. CXXVI. 5), which communicate with the external surface by a special organ (fig. CXXVI. 4) that conveys water into the interior of the body (fig. CXXVI. 5). By means of these ramifying water-tubes, upon the delicate walls of which the blood-vessels are spread out in minute and beautiful capillaries, the water is brought into immediate contact with the vascular system. [Illustration: Fig. CXXVI.—_Holothuria._