HYDROZOA, a class of Ccelenterata (q.v.) embracing the polyps, all of which bear a general resemblance to Hydra (q.vd. There are two alternating generations, that is, (1) the sessile asexual polyp, which gives rise to (2) a jelly fish or medusa. The hydroid polyp is like hydra, a two-layered vase-like sac, with a circle of tentacles around the mouth. This gives off by a budding process a bell-shaped medusa, which is much more highly organized than the polyp, having a well-developed digestive and nervous system, and sense-organs (eyes and ears, or otocysts). They have the peculiar cells known as Nematocysts (q.v.), which are numer ous in the tentacles, and secrete a fluid resem bling formic acid, which may be exploded with danger to their enemies. The Hydrozoa are cnidaria capable of producing two different types of individual, the polyp and the medusa. (See Goaco). Each of these develops from the egg through the blastula, porenchymula, gastrula and actinuala. The Hydrozoa are at
present divided into seven orders, the most im portant of which are, besides Hydraria repre sented by hydra: the Hydrocorallince, of which Millepora (q.v.) is the type; the Tubularia, comprising Hydractinia, Tubularia, etc.; the Campanurari, of which Campanularia, Clytia and °bolo are examples. Near this group be longs the extinct order of Graptolites, which were floating forms living in the Paleozoic seas. The last order (Stphonophora) comprises the Portuguese man-of-war (q.v.) and other forms, which are beautiful transparent pelagic animals, very brightly colored and highly specialized. An interesting type is the Actinozoa and the subclasses Zoontharia and Aleyosaria. Consult Hickson, and Ctenophora' (in (Cambridge Natural History,' Vol. I, 1906). See JELLYFISH ; POLYP.