The Bryozoa resemble living moss, and are found both in fresh and salt water. When observed under the microscope, this moss is seen to be composed of minute tubes, in which the Paludicella, a Bryozoon (Fig. 37), lives. Though this creature is small, it is more complex than many of the animals we have called attention to. The Paludicella has a mouth, gullet, stomach, and intestine, which are entirely shut off from the general cavity of the body; a great advance in structure as compared to that of the Anemone and Jelly-fish. The Bryozoa are usually classed with the Clams and Oysters (Mollusca); but, from their development from worm-like embryos, they are more justly considered as a group of worms. This view of the position of the Bryozoa is confirmed by the recent discoveries of the worm-like development of the Brachiopoda, the first class of the Mollusca. The Bryozoa are probably the root of the Mollusca, and the connecting link between them and the Worms. The Tunicata, the other division of the Sac-worms, are so called from the animals representing them being inclosed in a bag or tunic. They are a very important group, as showing probably that point where the stem of the Fishes originated. The young Ascidian (Fig. 38, a), one of the Tunicata, resembles a tadpole, and in this condition has quite as much of a backbone (Fig. 38, C) as the Amphioxus (Fig. 39, 40, C), the simplest vertebrate known. The Ascidian, when mature, is like a double-necked vase. The arrangement of the nervous system in the Tunicata differs from that of the Bryozoa, and serves as a distinguishing mark. The curious worm Sagitta (Fig. 36), the only representative of the Chaetognathi, has certain affinities with the thread-worm as well as with the simplest of the vertebrata; it is therefore placed between the two.
The Coelenterata are characterized by the want of specialized structures; that division of labor, so conspicuous in the higher animals, begins to be seen in the Worms, the digestive system in them being more or less developed, together with a rudimentary heart, respiratory and excretory apparatus, and the elements of a nervous system. The Tree of Worms is essentially an intermediate one,—its roots intimately connected with the simplest forms of life, Gregarinae, Infusoria, etc.,—its branches expanding into the
