To the lowest sub-kingdom or cyclo-neurose division belong five classes of animals ; viz., 1. Polygastrica, microscopic, simple, transpa rent, soft, aquatic animals, in which no nervous filament has yet been detected, generally pro vided with eyes, with a circular exsertile dental apparatus around the mouth, and with vibratile cilia for respiration and progressive motion, and provided with numerous internal stomachs or caeca communicating with the alimentary cavity. (See POLYGASTRICA.) 2. Porifera, simple, aquatic, soft, animals, without perceptible nervous or muscular fila ments or organs of sense, with a fibrous internal skeleton sometimes supported with silicious and sometimes with calcareous spicula, the body permeated with a soft gelatinous flesh, covered externally with minute absorbent pores, tra versed by numerous ramified anastomosing canals, which commence from the pores and terminate in large open vents, as seen in the annexed figure of the haling papillaris, Gr. (fig. 29), which represents the animal as alive, under water, with the usual currents passing inwards through its pores (a a), traversing its internal canals (b), and escaping by the larger vents (c, d.) (See PORIFERA.)
3. Polypifera, aquatic animals, of a plant like form, generally fixed, of a simple internal structure, for the most part without perceptible nerves or muscles, or organs of sense, and nou rished by superficial polypi, which are developed from the fleshy substance of the body, as in the canipanularia dichotoma, (fig. 30), where the irritable fleshy tubular portion of the animal is seen to occupy the interior of the base, the stem, and the branches, and to extend in the form of polypi from the open terminal cells. The polypi of most zoophytes are provided with tentacula around their orifice, as seen at B, (fig. 31), and the margins of these ten tacula are generally furnished with numerous minute processes, termed cilia, (see CILIA,) by the rapid vibration of which, currents are pro duced in the surrounding water for the pur pose of attract ing food and of aerating the surface and fluids of the body, as repre sented in fig.
3, A. (See Po