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.[1] 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.)
MEDWAY, a river in the south-east of England. It rises in the Forest Ridges, S.W. of East Grinstead in Sussex, and, increased by many feeders from these picturesque hills, has an easterly course to the county boundary, which it forms, turning northward for a short distance. Entering Kent near Ashurst,
its course becomes north-easterly, and this direction is generally maintained to the mouth. The river passes Tonbridge, receiving the Eden from the west, and later the Teise and Beult from the
south and east, all these streams watering the rich Weald (q.v.) to the south of the North Downs. These hills are breached by
the Medway in a, beautiful valley, in which lies Maidstone,
generally much narrower than the upper. valley. The characteristic
structure of this part of the valley is considered under the
heading Downs. Below Maidstone the valley forms a perfect
basin, the hills descending upon it closely above Rochester.
Below this city the river enters a broad, winding estuary, passing
Chatham, and at Sheerness joining that of the Thames, so that
the Medway may be considered a tributary, and its drainage area
of 680 sq. m. reckoned as part of that of the greater river.
The length of the Medway is about 60 m., excluding its many
lesser windings. The estuary is navigable for sea-going vessels
drawing 24 ft. up to Rochester Bridge. A considerable traffic
is carried on by small vessels up to Maidstone, and by barges up to
Tonbridge, the total length of the navigation being 43 m. The
marshy lowlands along the course of the river have yielded extensive
remains of Roman pottery, a plain ware of dark slate-colour.
MEEANEE, or Miani, a village in Sind, India, on the Indus
6 m. N. of Hyderabad. Pop. (1901), 962. It is famous as the
scene of the battle in which Sir Charles Napier, with only
2800 men, broke the power of the mirs of Sind on the 17th of
February 1843. The result of this victory was the conquest
and annexation of Sind.
MEEK, FIELDING BRADFORD (1817–1876), American
geologist and palaeontologist, the son of a lawyer, was born at Madison, Indiana, on the 10th of December 1817. In early life he was in business as a merchant, but his leisure hours were devoted to collecting fossils and studying the rocks of the neighbourhood
of Madison. Being unsuccessful in business he turned
his whole attention to science, and in 1848 he gained employment on the U.S. Geological Survey in Iowa, and subsequently in Wisconsin and Minnesota. In 1852 he became assistant to Professor James Hall at Albany, and worked at palaeontology with him until 1858. Meanwhile in 1853 he accompanied Dr F. V. Hayden in an exploration of the “Bad Lands” of Dakota and brought back valuable collections of fossils. In 1858 he went to Washington, where he devoted his time to the palaeontological work of the United States geological and geographical surveys, his work bearing “the stamp of the most faithful and conscientious research,” and raising him to the highest rank as a palaeontologist. Besides many separate contributions to science, he prepared with W. M. Gabb (1839–1878), two volumes on the palaeontology of California (1864–1869); and also a Report on the Invertebrate Cretaceous and Tertiary Fossils of the Upper Missouri Country (1876). He died at Washington, on the 22nd of December 1876.
- ↑ 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.
