The tentacles, intestines, and other organs constitute the “polypide,” the cell being simply the protective house formed by the latter. The body-cavity, which contains fluid, is in direct communication with the interior of the tentacles, which are hollow, and which act as respiratory organs by bringing the fluids of the body-cavity in proximity to the water. In _Flustra_ the body-cavities of the cells are shut off from each other, but pores and sieves in the partition walls allow of the junction of the inner linings of these cavities. The male and female reproductive elements are formed in the body-cavity. The egg develops in a helmet-shaped brood-pouch, the ovicell, situated at the summit of the cell and almost immersed in the cell above. The ciliated embryo swims about for a few hours and settles down to form the first polypide and cell; from the latter there arise buds which remain attached, and produce other buds, till a colony like that of _Flustra_ results. Among the ordinary cells are certain smaller cells (Fig. 1, _a_) slightly raised above the general level, different in shape from the ordinary kind and with thicker lids. These peculiar cells are termed avicularia, and chiefly contain muscles for opening and shutting the lid. They arise by modification of the ordinary cells, whereby all the organs of the polypide have become atrophied except the muscles. The Polyzoa[12] were so named by Vaughan Thompson, who, in 1820, discovered that certain plant-like animals, which had previously been classed with the zoophytes, possessed a much higher organisation, in that the intestine was separate from the body-cavity and not continuous with it as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the group Bryozoa[13] or Moss Animals. With the exception of one genus (_Loxosoma_), all Polyzoa form colonies, which arise by the continual budding of the cells, the buds remaining attached to the parent cells. The colonies vary endlessly in form and habit, occurring as crusts on rocks, etc., masses, broad fronds, branching tree-like growths, bushy tufts, etc. The texture and consistency may be gelatinous, cartilaginous, horny and flexible, or stony. The great majority of species are marine, but a considerable number inhabit fresh water. The Polyzoa are classified as follows:—