HY'DROZO'A (Neo-Lat. nom. pl., from Gk. Mnp, hyd6r, water + Nov, noon. animal). A class of Ccelenterata in which the simplest form of the body, as in the polyps, is a sae attached by its posterior cud, and with a digestive cavity communicating with the exterior by a mouth only. The mouth is surrounded by hollow ten tacles. The body is composed of two cellular layers, the ectoderm and endoderm, separated by a gelatinous. non-eellular •tnesogIcea.' The body is usually differentiated into two sorts of zo oids, i.e. 'polyps' or nutritive zooids, which are usually sexless. and 'medusa,' or reproductive zooids. The latter produce medusa buds, which on being set free are called medusa., and are bisexual. In these medusa the body is greatly more complicated than in the polyps. being bell or umbrella shaped, with a well-developed ner vous system, composed of a thread-ring around the eye of the disk and with ganglia near the `eyes' or sense organs. The digestive cavity is differentiated into a central and a peripheral portion, the latter forming radial and circular canals. The eggs and sperm are discharged ex ternally, and fertilization of the egg takes place in the sea. Nettling organs (see are usually present; they arise from some of the interstitial cells, and are most abundant in the tentacles. Many Hydrozoa (Hydra and Hydrae tinia excepted) exhibit the phenomenon of alter nation of generations (q.v.). The asexual polyps give rise to buds which develop into medusa., and the latter lay the eggs from which are batched the polyps. The medusa are more or less phos phorescent. They abound in all seas, while Hydra is a minute fresh-water form. Very primitive fresh-water forms are Protohydra and "Alicro hydra. which are without The llydrozoa are grouped in six orders: (1) L(piolirae, having a fixed zoiiphyte or polyp stage, and the sense organ. cetodermal (An thomedusa. and Leptomolusit.) ; Trarhylina , having no fixed stage, all being loco motive medusa• (Traellyincilusa• and Naretone dusa.) : (3) ydrueoruhlino, coral•making polyp: (Mill:vont and Stylaster) : ( I ) iph oi oil Ora pelagic hydrozoans in polymorphic colonies; (5) Gruplolithoifb a, an extinct Paleozoic group, re lated to (6) the llyrlrounJu.srr. The are a very heterogeneous assemblage of polyp and medusa forms, arranged in several families. In some species find only hydroids, in others only medusa.: but in many the hydroids and are alternate generations of It coat mon stock.
Of the orders above mentioned. the Hydro corallina and Oraptolithoidea are of importance to the geologist, for they contain fossils of con siderable interest. (See S•rtoxi.vroemt.A. and ( On.% r•om.rv.. ) Fossil represen to t Ives of the Scyphomedusze are rare. but have been described from Cambrian, Jurassic, and other formations.
Consult: Parker and Haswell. Te.rt•Itook of Zoology (London and New York, IS:17) ; Iluxley, The ()rennin Ilydro:ou (London, S59) ; Fowler, "The Hydromedusithe and the Se•ilioniciltisidie," in Lankester's Treatise on Zoi:loyy, part ii. (Lun dell, I 900 ). See CORAL; CTENOPII ORA ; II YMCA; IlYPROID ; I EDUS.E ; POLYP; SI PHON OPII OR ; et•.
HYENA, 111-'nil (Lit. hycraa, from Gk. tiama, hinoina, hyena, from r,•, Lys, hog, Lat. sus, AS., 011(:. mi. Cyr. Sun, Eng. sou• .\v. inn, pig: con nected either with Skl. sfi, to hear, or with Skt. sakura,swini‘.,a,inaker,so called from the grunt ). One of a family of carnivorous nab nulls remarkable for their generalized structure, which unites features characteristic of the eats, the civets, and the dogs. Their dentition is cal eulated for great tearing and crushing power, and includes six incisors and two canine teeth in cosh jaw. lice molars on each silt- of the upper jaw, and four ill the linder. This dentition and other features plan then) between the dogs and the civets. with the latter of which they arc con nected by Proteles. (See AAllo-\VOI.F.) They seize an objeet with so firm a hold that among. .\rahs they are proverbial for obstinaey. The Vert Of the neck sometimes Leconte anch•losed in old hyenas. In size they equal the largest logs. The hind quarters are lower and weaker than the fore quarters of the body. so that hyenas move with a shambling gait. The holly covered with 1.og coarse hair, forming a mane along the neck and hack. The feet have each four toes. The claws are strong.. fit for digging. and not retractile. The tail is short. Beneath the anus is a deep glandular pouch. contributing much to the offensive odor which is one of their litany disagreeable charaeteristies. Hyenas eat carrion as well as killed prey, and are of much use, like vultures, as scavengers, clear ing away the last remnants of carcasses. They sometimes attack cattle, especially if they lice, but rarely man, though they sometimes seize children. gives surprising instances of their boldness and strength ill East Africa. Dur ing the day they hide themselves in eaves, old rock tombs, and ruined edifices; by night they roam singly or in packs. They prowl about towns and villages. and often dig up corpses that have not been deeply Muriel. This, together with their aspect. and manners, has caused them to be gen erally regarded Nt iIII horror, and very exaggerated aecounts of their fierceness have been prevalent. Instead of being untamable. as was long the popular belief, they are capable of being com pletely tamed. and show an attachment to man similar to that of the dog; they have even been used as watch-dogs. Hyenas are found only in Africa and in Asia.