Page:EB1911 - Volume 18.djvu/88

 well on ordinary soils. It produces the best fruit in rich, loamy, somewhat moist ground. The tree may be grown as a standard, and chiefly requires pruning to prevent the branches from rubbing each other. The fruit should be gathered in November, on a dry day, and laid out upon shelves. It becomes “bletted” and fit for use in two or three weeks. The Japanese medlar is Eriobotrya japonica (see ), a genus of the same tribe of Rosaceae.

 MÉDOC, a district in France adjoining the left bank of the Gironde from Blanquefort (N. of Bordeaux) to the mouth of the Gironde. Its length is about 50 m., its breadth averages between 6 and 7 m. It is formed by a number of low hills, which separate the Landes from the Gironde, and is traversed only by small streams; the Gironde itself is muddy, and often enveloped in fog, and the region as a whole is far from picturesque. Large areas of its soil are occupied by vineyards, the products of which form the finest growths of Bordeaux. (See .)

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 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 (q.v.), the other type being the (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&#8202;2.). The exumbral nerve-ring is the larger and supplies the tentacles; the subumbral ring supplies the velum.

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.

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 (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