HYDROID (front Gk. i'Apar-aSig, hydroci1 s, like water, from hyd6r, water cidos, form). one of a class of ecelenterate ani mals, notable for their delicacy and beauty, and receiving their name from their structural re semblance to hydra (q.v.). The name is now generally restricted to the polyp forms of the Ilydromedusse. (See llY 11110Z0A . 1 They exist in compound colonies, one kind having the office of feeding the community, another of protecting it, and another of reproduction. The feeding hydroids are usually fixed. or attached to some object, and proceed from eggs of the reproduc tive hydroids, or medusa-, the latter in turn grow ing from buds produced by the former. The medusse sometimes remain attached to the stem, or become free-swimming. The body of the nu tritive hydroid is usually supported by a stem of variable length, but may rest immediately upon the bottom. From one individual buds appear and produce branching colonies of hundreds or thousands, often having a height of fifteen or twenty and the giant hydroid of Japan is more than three feet high. The reproductive hy droids are sometimes developed into perfect inedusse before leaving the parent. stem, hut they usually break away before attaining their per fect state. Some buds never hecome much de
veloped, and are called gonophores. These usual ly remain attached, but attain sexuality and re produet ive power. hydroids are covered with a chitinons envelope. which is continuous over the branching stem of the entire colony. hut some species are naked, and soft. In tubularian hydroids, the chitinous envelope, when present. simply inclose: the stem and branches. but is not expanded around the individual polyps lity drariths) in the form of a cup. In rian hydroids, however, each hydranth is sur rounded by a cup. a continuation of the chiti nous envelope, into which it can withdraw itself. Hydroids abound in the ocean in various parts of the world. notably the northwest coast of Amer ica. the Caribbean Sea and around Australia. Their colors are usually not brilliant, brown, flesh-color, and white being the most common. A familiar and beautiful example is the Portu guest- man-of-war (q.v.). Consult Agassiz, North merican ..tcalepho (Nlusetint of Comparative Zo;;Iogy, Cambridge, I S75 I. See S11.110N0P1 !ORA , A LTERN AT ION OF ( ; ENERATION S, and the accom panying