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polyp (q.v.). In a typical medusa we can distinguish the following parts. The umbrella-like body bears a circle of tentacles at the edge, whereby the body can be divided into a See also:

CONVEX

convex exumbrella or exumbral See also:

SURFACE

surface and a See also:

CONCAVE

concave subumbrella or subumbral surface. The vast See also:

MAJORITY (Fr. majorite; Med. Lat. majoritas; Lat. major, greater)

majority of jelly-See also:

fish See also:

FLOAT (in O. Eng. floc and flota, in the verbal form f eotan; the Teutonic root is flut-, another form of flu-, seen in " flow," cf. " fleet "; the root is seen in Gr. a-M e, to sail, Lat. pluere, to rain; the Lat, fluere and fluctus, wave, is not connect

float in the sea, with the exumbrella upwards, the subumbrella downwards. A few See also:

SPECIES

species, however, attach themselves temporarily or permanently to some See also:

FIRM

firm See also:

OBJECT

object by the exumbral surface of the body, and then the subumbral surface is directed upwards. From the centre of the subumbral surface hangs down the manubrium, like the handle of an umbrella or the clapper of a bell, bearing the mouth at its extremity. In addition to the tentacles, the margin of the umbrella bears sense-organs, which may be of several kinds and may attain a high degree of complexity. Medusae capture their prey, consisting of small organisms of various kinds, especially See also:

CRUSTACEA

Crustacea, by means of the tentacles which hang out like fishing-lines in all directions.

When the prey comes into contact with the tentacles it is paralysed, and at the same See also:

time held firmly, by the barbed threads shot out from the stinging organs or nematocysts. Then by contraction of the tentacles the prey is See also:

DRAWN

drawn into the mouth. Medusae thus form an important constituent of the See also:

PLANKTON

plankton or floating See also:

FAUNA

fauna of the ocean, and compete with fish and other animals for the See also:

food-See also:

SUPPLY (through Fr. from Lat. supplere, to fill up)

supply furnished by minuter forms of See also:

LIFE

life. A medusa has a layer of muscles, more or less strongly See also:

DEVELOPED

developed, See also:

RUNNING

running in a circular direction on the surface of the subumbrella, the contractions of which are antagonized by the See also:

ELASTICITY

elasticity of the gelatinous substance of the body. By the See also:

con-See also:

TRACTION (Lat. trahere, to draw)

traction of the subumbral circular muscles the concavity of the subumbrella is increased, and as See also:

WATER

water is thereby forced out 'of the subumbral cavity the See also:

animal is jerked upwards. In this way jelly-fish progress feebly by the pumping movements of the umbrella. Besides the circular subumbral muscles, there may be others running in a radial direction, chiefly developed as the See also:

LONGITUDINAL

longitudinal retractor muscles of the manubrium. In some cases the circular subumbral muscles form a rim known as the velum (v., see fig. 1), projecting into the subumbral cavity just within the See also:

RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring of marginal tentacles. The two See also:

PRINCIPAL

principal I The See also:

GOOSEBERRY

gooseberry-like or band-shaped jelly-fishes belong to the class See also:

CTENOPHORA

Ctenophora (q.v.). divisions of the medusae are characterized by the presence or See also:

ABSENCE (Lat. absentia)

absence of a velum. Correlated with the well-developed See also:

MUSCULAR

muscular See also:

SYSTEM

system and sense-organs of the medusa, we find also a distinct See also:

NERVOUS

nervous system, either, when there is no velum, in the. form of concentrations of nervous See also:

MATTER

matter in the vicinity of each sense-See also:

ORGAN

organ, or, when a velum is See also:

PRESENT

present, as two continuous rings running See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the margin of the umbrella, one See also:

EXTERNAL

external to the velum (exumbral See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-ring, n.r', see fig.

1), the other See also:

INTERNAL

internal to it (subumbral nerve-ring, n.See also:

R2(rt-s2)

r2.). The exumbral nerve-ring is the larger and Every possible variety of body-form compatible with the fore-going description may be exhibited by different species of medusae. The body may show modifications of form which can be compared to a shallow saucer, a See also:

CUP (in O.E. cuppe; generally taken to be from Late Lat. cuppa, a variant of Lat. cupa, a cask, cf. Gr. KuaeXXov)

cup, a bell or a See also:

THIMBLE

thimble. The marginal tentacles may be very numerous or may be few in number or even absent altogether; and they may be See also:

SIMPLE

simple filaments, or branched in a complicated manner. The manubrium maytde excessively long or very See also:

SHORT, FRANCIS JOB (1857– )

short, and in rare cases absent, the mouth then being flush with the subumbral surface. The mouth may be circular or four-cornered, and in the latter See also:

case the manubrium at the angles of the mouth may become drawn out into four lappets, the oral arms, each with a groove on its inner See also:

side continuous with the corner a See also:

DIAGRAM

Diagram of the structure of a medusa; the ectoderm is See also:

LEFT

left clear, the endoderm is dotted, the .mesogloea is shaded See also:

black; a-b, principal See also:

AXIS (Lat. for " axle ")

axis (see HYDROZOA) ; to the left of this See also:

LINE

line the See also:

section Is supposed to pass through an inter-See also:

RADIUS

radius (I.R.); to the right through a radius (R). The exumbral surface is uppermost, the subumbral surface, with the manubrium and mouth, is facing downwards. St. See also:

STOMACH (Gr. vrbsaxos from arbga, a mouth)

Stomach. G. Gonads. r.c.

Radial See also:

canal. n.r.' Exumbral (so-called c.c. Circular or ring-canal. upper) nerve-ring. e.l. Endoderm-lamella. n.r.2 Subumbral (so-called v. Velum. See also:

LOWER

lower) nerve-ring. (For other figures of medusae see HvuROZOA.) of the mouth. The oral arms are the starting-point of a further See also:

SERIES (a Latin word from serere, to join)

series of See also:

variations; they may be simple flaps, crinkled and folded in various ways, or they may be subdivided, and then the branches may simulate tentacles in See also:

APPEARANCE (from Lat. apparere, to appear)

appearance. In the genus Rhizostoma, See also:

COMMON

common on the See also:

BRITISH

British coasts and conspicuous on See also:

account of its large See also:

SIZE

size, the oral arms, originally distinct and four in number, undergo concrescence, so that the entrance to the mouth is reduced to numerous See also:

FINE

fine pores and canals? Like the external structure, the internal See also:

anatomy of the medusa shows a See also:

COMPLETE

complete radial symmetry, and is simple in See also:

plan but often complicated In detail (see fig. I). As in all Hydrozoa (q.v.) the body See also:

wall is composed of two See also:

CELL (from Lat. cella, probably from an Indo-European kal —seen in Lat. celare, to hide; another suggestion connects the word with Lat. cera, wax, taking the original meaning to refer to the honeycomb)

cell-layers; the ectoderm and endoderm. between which is a structureless gelatinous secreted layer, the mesogloea. As the name jelly-fish implies, the mesogloea is greatly developed and abundant in quantity.

It may be traversed bye processes of the cells of the ectoderm and endoderm, or it may contain cells which have migrated into it from these two layers. The ectoderm covers the whole external surface of the animal, while the endoderm lines the coelenteron or gastrovascular space; the two layers meet each other, and become continuous, at the edge of the mouth. The mouth leads at once into the true See also:

DIGESTIVE

digestive cavity, divisible into an oesophageal region in the ma.nubriurn and a more dilated cavity, the stomach (st.), occupying the centre of the umbrella. From the stomach, canals arise termed the radial canals (r.c.) ; typically four in number, they run in a radial direction to the edge 2 For other variations of the medusa, often of importance for systematic See also:

CLASSIFICATION (Lat. classis, a class, probably from the root cal-, cla-, as in Gr. icaMce, clamor)

classification, see See also:

HYDROMEDUSAE

HYDROMEDUSAE and See also:

SCYPHOMEDUSAE

SCYPHOMEDUSAE. of the umbrella. There the radial canals are joined by a ring-canal (c.c.) which runs round the margin of the umbrella. From the ring-canal are given off tentacle-canals which run down the axis of each tentacle; in many cases, however, the cavity of the tentacle is obliterated and instead of a canal the tentacle contains a solid core of endoderm. See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

Oesophagus, stomach, radial canals, ring-canal and tentacle-canals, constitute together the gastrovascular system and are lined throughout by endoderm, which forms also a See also:

FLAT (a modification of O. Eng. flet, an obsolete word of Teutonic origin, meaning the ground beneath the feet)

flat See also:

SHEET

sheet of cells connecting the radial canals and ring canal together like a See also:

WEB (a word common to Teutonic languages, cf. Du. webbe, Dan. vaev, Ger. Gewebe, all from the Teutonic wabh—to weave)

web; this is the so-called endoderm-lamella (e.l.), a most important feature of medusan See also:

MORPHOLOGY, (Gr. yopdsil, form)

morphology, the nature of which will be apparent when the development is described. As a See also:

general See also:

RULE

rule the mouth is the only See also:

APERTURE (from Lat. aperire, to open)

aperture of the gastrovascular system; in a few cases, however, excretory pores are found on the ring-canal, but there is never any anal opening. The sense-organs of medusae are of two classes: (1) pigment spots, sensitive to See also:

LIGHT

light, termed ocelli, which may become elaborated into See also:

eye-like structures with See also:

lens, retina and vitreous body; (2) organs of the sense of See also:

BALANCE (derived through the Fr. from the Late Lat. bilantia, an apparatus for weighing, from bi, two, and lanx, a dish or scale)

balance or See also:

ORIENTATION

orientation, commonly termed otocysts or statocysts. The sense-organs are always situated at the margin of the unbrella and may be distinguished from the morphological point of view into two categories, according as they are, or are not, derived from.modifications of tentacles; in the former case they are termed tentaculocysts. (For See also:

fuller See also:

INFORMATION (from Lat. informare, to give shape or form to, to represent, describe)

information upon the sense-organs see HYDROMEDUSAE.) Medusae are nearly always of See also:

SEPARATE

separate sexes, and instances of hermaphroditism are rare.

The gonads or generative organs may be produced either in the ectoderm or the endoderm. When the gonads are endodermal, they are formed on the See also:

FLOOR (from O. Eng. flor, a word common to many Teutonic languages, cf. Dutch vloer, and Ger. Flur, a field, in the feminine, and a floor, masculine)

floor of the stomach; when ectodermal (G, see fig. 1), they are formed on the subumbral surface, either on the manubrium or under the stomach or under the radial canals, or in more than one of these regions. Medusae often have the See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power of budding, and the buds are formed either on the manubrium, or at the margin of the umbrella, or on an out-growth or " stolon " produced from the exumbral surface. The internal anatomy of the medusa is as variable as its external features. The mouth may See also:

lead directly into the stomach, without any oesophagus. The stomach may be situated in the disk, or may be drawn out into the See also:

BASE

base of the manubrium, so that the disk is occupied only by the radial canals. On the other See also:

hand the stomach may have lobes extending to the ring-canal, so that radial canals may be very short or absent. The radial canals may be four, rarely six, or a multiple of these See also:

numbers, and may be very numerous. They may be simple or branched. (For other anatomical variations see HYDROMEDUSAE and SCYPHOMEDUSAE.) In development the medusa can be derived easily by a See also:

PROCESS

process of See also:

DIFFERENTIAL

differential growth, combined with concrescence of cell-layers, from the actinula-larva. (For figures see HYDROZQA.) The actinula is polyp-like, with a See also:

SACK

sack-like or rounded body; a See also:

CROWN

crown of tentacles surrounds a wide peristome, in the centre of which is the mouth, usually raised on a conical process termed the hypostome.

To produce a medusa the actinula grows greatly along a See also:

PLANE

plane at right angles to the See also:

VERTICAL (from Lat. vertex, highest point)

vertical axis of the body, whereby the aboral surface of the actinula becomes the exumbrella, and the peristome becomes the subumbrella. The crown of tentacles thus comes to form a fringe to the margin of the body, and the hypostome becomes the manubrium. As a result of this See also:

CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change of form the gastric cavity or coelenteron becomes of compressed lenticular form, and the endoderm lining it can be distinguished as an upper or exumbral layer and a lower or subumbral layer. The next event is a great growth in thickness of the gelatinous mesogloea, especially on the exumbral side; as a result the flattened coelenteron is still further compressed so that in certain spots its cavity is obliterated, and its exumbral and subumbral layers of endoderm come into contact and undergo concrescence. As a rule four such areas of concrescence or calhanzmata (E. See also:

HAECKEL, ERNST HEINRICH (1834- )

Haeckel) are formed. The cathammal areas may remain very small, See also:

mere See also:

WEDGE (O. Eng. wecg, a mass of metal, cognate with Dutch wig, wigge, Dan. vaegge, &c.; in Lith. the cognate form outside Teut. is found in wagis, a peg, spigot; there is no connexion with " weigh," " weight," which must be referred to the root wegh, to li

wedge-shaped partitions dividing up the coelenteron into a four-lobed stomach, the lobes of which communicate at the periphery of the body by a spacious ring-canal. More usually each cathamma is a wide triangular See also:

AREA

area, reducing the peripheral portion of the coelenteron to the four narrow radial canals and the ring-canal above described. The two apposed layers of endoderm in the cathammal area undergo complete See also:

FUSION

fusion to form a single layer of epithelium, the endoderm-lamella of the adult medusa. Medusae, when they reproduce themselves by budding, always produce medusae, but when they reproduce by the sexual method the embryos produced from the See also:

egg' grow into medusae in some cases, in other cases into polyps which bud medusae in their turn. In this way complicated cycles of alternating generations arise, which are described fully in HYDROMEDUSAE and SCYPHOMEDUSAE. Medusae are exclusively aquatic animals and for the most See also:

PART

part marine, but at least two fresh-water species are known' Limnocodium sowerbyi was first discovered See also:

SWIMMING (from " swim," A.S. swimman, the root being common in Teutonic languages)

swimming in the tank in which the water-See also:

LILY

lily, See also:

Victoria regia, is cultivated in See also:

Kew Gardens, and 'C.

L. Boulenger (Prot. Zool. See also:

Soc. of See also:

LONDON

London, 1907, p. 516) recorded the See also:

DISCOVERY

discovery of a third species by himself and W. A. Cunnington, in the brackish water of See also:

lake Birket el Kerun in the See also:

EGYPTIAN

Egyptian See also:

FAYUM

Fayum.has since been found sporadically in a similar situation in other botanical gardens, its most See also:

RECENT

recent appearance being at See also:

LILLE

Lille. These jelly-fishes are probably budded from a See also:

minute polyp-stock introduced with the roots of the lily. Another fresh-water form is Limnocnida tanganyicae, discovered first in lake See also:

TANGANYIKA (a name said by V. L. Cameron to signify a " mixing-place ")

Tanganyika, and now known to occur also in the Victoria See also:

NYANZA (from the ancient Bantu root word anza, a river or lake)

Nyanza and in the See also:

Niger. A medusa with a remarkable See also:

HABIT (through the French from Lat. habitus, from habere, to have, hold, or, in a reflective sense, to be in a certain condition; in many of the English senses the French use habitude, not habit)

habit of life is Mnestra parasites, which is parasitic on the pelagic mollusc Phyllirrhoe, attaching itself to the See also:

HOST

host by its subumbral surface; its tentacles, no longer required for obtaining food, have become rudimentary. A parasitic mode of life is also seen in medusae of the genus Cunina during the larval See also:

CONDITION (Lat. condicio, from condicere, to agree upon, arrange; not connected with conditio, from condere, conditum, to put together)

condition, but the habit is abandoned, in this case, when the medusae become adult. For figures of medusae see (i) E.

Haeckel, " Das System der Medusen," Denkschriften med-natwiss. Ges. See also:

JENA

Jena (1879, 2 vols.); (2) Id., " Deep-Sea Medusae," Challenger Reports, See also:

ZOOLOGY (from Gr. Nov, a living thing, and ?byos, theory)

Zoology, IV. pt. ii. (1882) ; (3) O. See also:

MAAS, JOSEPH (1847-1886)

Maas, " See also:

Die craspedoten Medusen," Ergebn. Plankton-Expedition, II. (1893) ; (4) id., " Die Medusen," Mem. See also:

Mus. Comp. Zool. Harvard, See also:

XXIII

XXIII. (1897); (5) G.

J. See also:

ALLMAN, GEORGE JAMES (1812-1898)

Allman, "A Monograph of the Gymnoblastic or Tubularian Hydroids," Pay. Soc. (1871-1872). (E. A.

End of Article: MEDUSA

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