Like the preceding sub-kingdom--the _Echinodermata_--worms possess a system of water tubes; this system, however, is not in any way connected with the function of locomotion, but is, in many cases at least, if not in all, intimately associated with the process of respiration. It consists of a series of tubes, arranged in pairs in the successive segments, communicating with the body-cavity internally, and opening at the exterior by means of pores in the cuticle. In some there is a highly organised system of bloodvessels, containing blood that is usually either colourless, red, or green, but the colour of the blood is never due to the presence of corpuscles, as is the case with higher animals, the tint being due to the plasma or fluid portion of the blood; and though worms cannot be said to possess a true heart, yet they often have one or more contractile bloodvessels which serve the purpose of propelling the blood. Most worms possess a nervous system, and, where this is present, it consists of a chain of ganglia, placed along the ventral side of the body, beneath the digestive tube, all united by means of a nerve cord, and distributing nerves in pairs to various parts of the body; and it may be well to note here one very important point of distinction between the general arrangement of the central portion of the nervous system in the worms and higher invertebrates, as compared with that of the corresponding structure in the vertebrates:--In the former the main axis of the system, consisting, as we have seen, of a chain of ganglia connected by a nerve cord, is invariably placed along the _ventral_ portion of the body-cavity--the surface on which the animal crawls; while in the vertebrates the axis of the nervous system lies along the upper or dorsal part of the body; and, instead of lying in the general body-cavity, in company with the organs of digestion and circulation, is enclosed in the bony canal formed by the vertebral column. It will be seen from this that when it is desired to examine the nervous system of the invertebrate animal, the body-wall should be opened along the middle of the ventral surface, while, in the vertebrate, the central axis should be exposed from above.