ACALEPH'. (Gr. " nettles"), a term given by Aristotle to the jelly-fishes or medusidee and their allies, in allusion to their stinging propensities. As in all other calenterate animals, the iirticating or stinging properties of such forms reside in the cnithe or "thread cells," with which the tissues of their bodies are provided. These cells consist each of a sac or vesicle, containing fluid and a thread-like filament; the cell rupturing on being pressed or otherwise irritated, and emitting the thread and fluid. The former must act mechanically as a kind of dart; whilst the fluid acts chemically in effects by its injection into the wound made by the filament. Some of the forms allied to the jelly-fishes, and included under the old term acalephm—such as the physalico or "Portuguese men-of-war "—sting, by means of these cells, so severely that the effects on the human subject may persist for days or even weeks.
In modern zoology, it may be noted, the term acalepho3 is now generally abolished. Formerly, this name was given to a group of coelenterate or radiate animals, repre sented by the true medusido3 or jelly-fishes, and also by the lucernarida; whilst older systems still, included in the group acalephm other oceanic organisms (ca/yrophoridce and phrophoridce) among which were the "Portuguese man-of-war," etc., and also the order ctenophora (bero'e, cesium veneris, etc.), this latter order being now removed to a class superior to that of the jelly-fishes and their allies. In modern systems of zoologi cal classification, therefore, the old division of the acalephie is represented by at least two distinct orders of coelenterate animals. Thus the true jelly-fishes or medusidic con stitute the sub-class discophora (" disk-bearers"), and are distinguished by being free swimming forms; the body in each consisting of a single organism, and being composed of a clear gelatinous swiinmin,g-bell or nectocalyx, from the roof of which the mouth is suspended; whilst throughout the substance of the bell-shaped body a system of radial and circular canals is distributed. These organisms, familiar to every sea-side visitor, swim
gracefully by contracting and expanding their clear jelly-like bodies; the aperture or mouth of the bell being generally closed or protected by a membrane named the reit or velum. Around the margin of the bell auditory sacs or hearing-organs are found; and pigment-Spots or ocelli existing in the same situation are believed to represent rudimen tary eyes. Tentacles or organs of touch are also developed, and may,dcpend from the margins of the bell, rThe,true medusiche are named gytonoplithalm44,. or "naked-eyed" forms, to distinguish them from the lucernarida, in which the eyes are protected by a kind of "hood." The lucernarida accordingly are named by some zoologists steganop• thattnate ("covered - eyed ") inedusle; whilst, more properly, the latter term is limited to medusa-like forms included r within that order. some of these " medusre some vet') • peculiar reproductive phases occur. Y . \ Thus the egg of a medusa may be seen.". to give rise to a little rooted organism.
1 I like a little (q.v.) iu form, and which is named the This • kt • latter organism then becomes divided transversely into a number of saucer- like segments, and is named the stroblla BerGe. (sars); whilst, ultimately, the segmentsa a, tentaeuln: b mouth; become detached, each swimming away c. termination of intestine. PhySalla. as a young medusa, and being known as an ephyra. These animals feed on minute crustacea, fishes, and the like; and very many .exhibit a phosphorescent light, or animal luminosity.