1911 Encyclopædia Britannica/Medusa

by Edward Alfred Minchin

MEDUSA, the name given by zoologists to the familiar marine animals known popularly as jelly-fishes; or, to be more accurate, to those jelly-fishes^[1] in which the form of the body resembles that of an umbrella, bell or parachute. The name medusa is suggested by the tentacles, usually long and often numerous, implanted on the edge of the umbrella and bear the stinging organs of which sea-bathers are often disagreeably aware. The tentacles serve for the capture of prey and are very contractile, being often protruded to a great length or, on the other hand, retracted and forming corkscrew-like curls. Hence the animals have suggested to vivid imaginations the head of the fabled Gorgon or Medusa with her chevelure of writhing snakes.

The medusa occurs as one type of individual in the class Hydrozoa (q.v.), the other type being the 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 convex exumbrella or exumbral surface and a concave subumbrella or subumbral surface. The vast majority of jelly-fish float in the sea, with the exumbrella upwards, the subumbrella downwards. A few species, however, attach themselves temporarily or permanently to some firm 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 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 time held firmly, by the barbed threads shot out from the stinging organs or nematocysts. Then by contraction of the tentacles the prey is drawn into the mouth. Medusae thus form an important constituent of the plankton or floating fauna of the ocean, and compete with fish and other animals for the food-supply furnished by minuter forms of life.

A medusa has a layer of muscles, more or less strongly developed, running in a circular direction on the surface of the subumbrella, the contractions of which are antagonized by the elasticity of the gelatinous substance of the body. By the contraction of the subumbral circular muscles the concavity of the subumbrella is increased, and as water is thereby forced out of the subumbral cavity the 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 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 ring of marginal tentacles. The two principal divisions of the medusae are characterized by the presence or absence of a velum.

Correlated with the well-developed muscular system and sense-organs of the medusa, we find also a distinct nervous system, either, when there is no velum, in the form of concentrations of nervous matter in the vicinity of each sense-organ, or, when a velum is present, as two continuous rings running round the margin of the umbrella, one external to the velum (exumbral nerve-ring, n.r¹, see fig. 1), the other internal to it (subumbral nerve-ring, n.r ².). The exumbral nerve-ring is the larger and supplies the tentacles; the subumbral ring supplies the velum.

Fig. 1. Diagram of the structure of a medusa; the ectoderm is left clear, the endoderm is dotted, the mesogloea is shaded black; a-b, principal axis (see Hydrozoa); to the left of this line the section is supposed to pass through an inter-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.	Stomach.	G.	Gonads.
r.c.	Radial canal.	n.r.1	Exumbral (so-called
c.c.	Circular or ring-canal.		upper) nerve-ring.
e.l.	Endoderm-lamella.	n.r.2	Subumbral (so-called
v.	Velum.		lower) nerve-ring.
(For other figures of medusae see Hydrozoa.)

Every possible variety of body-form compatible with the foregoing 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 cup, a bell or a thimble. The marginal tentacles may be very numerous or may be few in number or even absent alto ether; and they may be simple filaments, or branched in a complicated manner. The manubrium may be excessively long or very 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 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 side continuous with the corner of the mouth. The oral arms are the starting-point of a further series of 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 appearance. In the genus Rhizostoma, common on the British coasts and conspicuous on account of its large size, the oral arms, originally distinct and four in number, undergo concrescence, so that the entrance to the mouth is reduced to numerous fine pores and canals.^[2]

Like the external structure, the internal anatomy of the medusa shows a complete radial symmetry, and is simple in plan but often complicated in detail (see fig. 1). As in all Hydrozoa (q.v.) the body wall is composed of two 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 by 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 digestive cavity, divisible into an oesophageal region in the manubrium 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 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. Oesophagus, stomach, radial canals, ring-canal and tentacle-canals, constitute together the gastrovascular system and are lined throughout by endoderm, which forms also a flat sheet of cells connecting the radial canals and ring canal together like a web; this is the so-called endoderm-lamella (e.l.), a most important feature of medusan morphology, the nature of which will be apparent when the development is described. As a general rule the mouth is the only 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 light, termed ocelli, which may become elaborated into eye-like structures with lens, retina and vitreous body; (2) organs of the sense of balance or 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 fuller information upon the sense-organs see Hydromedusae.)

Medusae are nearly always of 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 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. Medusæ often have the power of budding, and the buds are formed either on the manubrium, or at the margin of the umbrella, or on an outgrowth or “stolon” produced from the exumbral surface.

The internal anatomy of the medusa is as variable as its external features. The mouth may lead directly into the stomach, without any oesophagus. The stomach may be situated in the disk, or may be drawn out into the base of the manubrium, so that the disk is occupied only by the radial canals. On the other 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 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 process of differential growth, combined with concrescence of cell-layers, from the actinula-larva. (For figures see Hydrozoa.) The actinula is polyp-like, with a sack-like or rounded body; a 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 plane at right angles to the vertical axis of the body, whereby the aboral surface of the actinula becomes the ex umbrella, and the peristome becomes the sub umbrella. The crown of tentacles thus comes to form a fringe to the margin of the body, and the hypos tome becomes the manubrium. As a result of this 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 cathammata (E. Haeckel) are formed. The cathammal areas may remain very small, mere 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 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 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 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 part marine, but at least two fresh-water species are known.^[3] Limnocodium sowerbyi was first discovered swimming in the tank in which the water-lily, Victoria regia, is cultivated in Kew Gardens, and has since been found sporadically in a similar situation in other botanical gardens, its most recent appearance being at Lille. These jelly-fishes are probably budded from a minute polyp-stock introduced with the roots of the lily. Another fresh-water form is Limnocnida tanganyicae, discovered first in lake Tanganyika, and now known to occur also in the Victoria Nyanza and in the Niger. A medusa with a remarkable habit of life is Mnestra parasites, which is parasitic on the pelagic mollusc Phyllirrhoe, attaching itself to the 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 condition, but the habit is abandoned, in this case, when the medusae become adult.

For figures of medusae see (1) E. Haeckel, “Das System der Medusen,” Denkschriften med-natwiss. Ges. Jena (1879, 2 vols.); (2) Id., “Deep-Sea Medusae,” Challenger Reports, Zoology, IV. pt. ii. (1882); (3) O. Maas, “Die craspedoten Medusen,” Ergebn. Plankton-Expedition, II. (1893); (4) id., “Die Medusen,” Mem. Mus. Comp. Zool. Harvard, XXIII. (1897); (5) G. J. Allman, “A Monograph of the Gymnoblastic or Tubularian Hydroids,” Ray. Soc. (1871–1872).  (E. A. M.) 

1. The gooseberry-like or band-shaped jelly-fishes belong to the class Ctenophora (q.v.).
2. For other variations of the medusa, often of importance for systematic classification, see Hydromedusae and Scyphomedusae.
3. C. L. Boulenger (Proc. Zool. Soc. of London, 1907, p. 516) recorded the discovery of a third species by himself and W. A. Cunnington, in the brackish water of lake Birket el Kerun in the Egyptian Fayum.