. Text book of zoology. Zoology. 180 Polyzoa. body-cavity, and at the other, to the exterior by a common aperture, near the lophophore * The Polyzoa usually have a large body-cavity, filled with a liquid in which amoeboid cells are found; it contains, besides the ali- mentary canal, a cord, the funiculus (Fig. 142), stretching from the stomach to the body-wall, upon which, or upon the inner side of the body-wall, ova and sperma- tozoa appear, both, usually, in the same individual; special sexual ducts are absent, the genital products (or embryos) pass out through holes in the body-wall, or thr
. Text book of zoology. Zoology. 180 Polyzoa. body-cavity, and at the other, to the exterior by a common aperture, near the lophophore * The Polyzoa usually have a large body-cavity, filled with a liquid in which amoeboid cells are found; it contains, besides the ali- mentary canal, a cord, the funiculus (Fig. 142), stretching from the stomach to the body-wall, upon which, or upon the inner side of the body-wall, ova and sperma- tozoa appear, both, usually, in the same individual; special sexual ducts are absent, the genital products (or embryos) pass out through holes in the body-wall, or through the excretory organs. G-enerally, the fertilised ovum undergoes its earliest development within the body of the parent, in many marine forms, in a special invagination of the body-wall (ooecium). Among the freshwater Poly- zoa reproduction is effected by means of statoblasts, as well as by fertilised ova. The statoblasts are small, discoid bodies arising upon the funiculus by a peculiar process of budding. They are produced chiefly towards the end of the, summer, and rest during the winter, developing, in the next year, into a new colony. Each is provided with a hard ornamental shell, in whose edge there are small air cavities. The new animal is formed from a mass of cells within. In many forms a very remarkable degeneration of the lophophore and alimentary canal occurs, constituting the so-called " brown body," from which these parts are, after a time, reconstructed. The colonies formed by the Polyzoa are of very different kinds. Some are much branched (Fig. 141), and either stand erect from, or creep over, some foreign object; others are laminate, lying upon the substratum or standing upright: or they may be more massive. The colony is almost always fixed; a single freshwater form {Cristatella) is free. Amongst many of the Chilostoma, dimorphism, like that in the Hydrozoa, occurs. Specially common among the ordinary individuals. Fig. 141. Plumatella pohimorpha,
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
