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Bible Encyclopedias
Scyphomedusae
1911 Encyclopedia Britannica
or or Acalephae, one of the two subdivisions of the Hydrozoa, the other being the Hydromedusae. The subclass Scyphomedusae contains a number of animals which in the adult condition are medusae or jellyfishes (see Medusa), exclusively marine in habitat and found in all seas. They are chiefly pelagic organisms, floating at or near the surface of the water, but occur also at great depths, and are sometimes fixed and sessile in habit. Many species attain a large size and by their brilliant coloration are very conspicuous objects to the mariner or traveller. In spite of the soft nature of their bodies, a number of Scyphomedusae have been found fossil; see especially Maas (7 and 12).
A scyphomedusa is distinguished from a hydromedusa chiefly by the following points. The umbrella has a lobed, indented margin, a character only seen amongst Hydromedusae in the order Narcomedusae, and it is without the characteristic velum of the Hydromedusae; hence the Scyphomedusae are sometimes termed Hydrozoa Acraspeda. The sense-organs are covered over by flaps of the umbrellar margin (hence " Steganophthalmata "), and are always tentaculocysts, that is to say, reduced and modified tentacles, which bear usually both ocelli and otocysts, and are hollow. The gonads are formed in the endoderm (hence " Entocarpeae "), and the generative products are shed into the gastric cavity and pass to the exterior by way of the mouth. The development from the egg may be direct, or may take place with an alternation of generations (metagenesis), in which a non-sexual individual, the so-called scyphistoma or scyphopolyp, produces by budding the sexual medusae.
Morphology of the Scyphomedusa
As already stated, a medusa of this order may be free-swimming or sessile in habit. Intermediate between these two types are species which have the power of temporal fixation by the exumbral surface. Such forms when undisturbed fix themselves to the bottom and rest with their mouths and tentacles uppermost. If disturbed they swim about like other medusae until a favourable opportunity presents itself for resuming the sedentary habit. A well-known example of a permanently sessile form is Lucernaria, common on the Atlantic coasts of Europe, especially in Zostera-beds, attached to the weed. It resembles in general appearance a polyp, lacking even the characteristic medusan sense-organs, which are present, however, in the allied genus Haliclystus (fig. I), proving its medusan nature beyond all doubt.
The body-form of the Scyphomedusae varies from that of a conical or roughly cubical cap (fig. 4), to that of a shallow saucer or disk (fig. 2a). The tentacles vary in number from four, the primitive I. From the side.
II. From above.
III. From the side, with the umbrella kl, drawn back and the mouth oc, thrust out.
IV. A tentaculocyst (" colleto-cysto- o, phore" or " marginal anchor ") se, seen from the subumbral side.
p, Stalk.
su, Subumbrella.
t, Knobbed tentacles in eight clusters.
ra, Tentaculocysts, four perradial, four interradial.
number, to a very large number, but in one suborder, the Rhizostomeae, tentacles are absent altogether (fig. 3, a). Typically the tentacles have the form of long flexible filaments, hollow or solid, implanted singly on the margin of the umbrella (fig. 3, b), but in some species they occur in groups or tufts (fig. 15), and in Lucernaria and its allies a bunch of small capitate tentacles is found on each of the eight adradial lappets of the margin (fig. I). A true velum is absent, as already stated, but in Charybdaea (fig. 4) a structure is found termed a velarium (Ve), which is a flap hanging down from the margin of the umbrella, and which consists of a fold of the subumbral ectoderm containing endodermal canals. A true velum, such as is found in Hydromedusae, never contains endoderm.
The mouth may be a simple structure at the extremity of the manubrium, or may be four-cornered, with the corners drawn out into so-called oral arms, each of which bears on the inner side a groove continuing the angle of the mouth (fig. 2a). In some genera the oral arms are of great length, and in the suborder Rhizostomeae they undergo concrescence to form a proboscis (fig. 3, a), in such a way that the mouth becomes nearly obliterated, and is reduced to a system of fine canals opening to the exterior by small pores.
The mouth leads into the spacious stomach, which is typically four lobed (fig. 2b, v). On the floor of the stomach are borne the conspicuous gonads (ov), and also tentacle-like processes termed gastric filaments or phacellae, projecting into the cavity of the stomach. The gonads are folds of the endoderm containing generative cells, and are primitively four in number, situated interradially, but each gonad may be divided into two by the partition which separates two adjacent lobes of the stomach, that is to say, by one of the areas of concrescence between exumbral and subumbral endoderm, whence arises a condition with eight gonads which is by no means uncommon. As a rule these medusae are of separate sexes, but hermaphrodite forms are known, for example, the conspicuous British (east-Atlantic) medusa Chrysaora (fig. 3, b). Immediately below each gonad the subumbral ectoderm is pushed in, as it were, to form a pit or deep cavity (fig. 2a, FIG. 2a. - Surface view of the Subumbrella or oral aspect of Aurelia aurita, to show the position of the openings of the subgenital pits, GP. In the centre is the mouth, with four perradial arms corresponding to its angles (compare fig. II). The four sub-genital pits are seen to be interradial. x indicates the outline of the roof (aboral limit) of a subgenital pit; y, the outline of its floor or oral limit, in which is the opening.
x, y) opening by a wide aperture (GP). These cavities are known as the infundibular or subgenital cavities. They serve probably for the aeration of the gonads by admitting to their vicinit y water with its dissolved oxygen; they never serve as genital ducts, since the generative products are always dehisced into the stomach and pass out by the mouth. In some genera, for instance, Cyanea and its allies the gonad as a whole protrudes through the subgenital cavity as if it had undergone a hernia, and hangs down in the subumbral space as if suspended by a mesentery (fig. is). Usually the four subgenital cavities are distinct from each other (so-called tetrademnic condition), but in many Rhizostomeae, for example, Crambessa, the subgenital cavities join together under the subumbral floor of the stomach (so-called monodemnic condition) and coalesce to form a so-called subgenital portico placed on the oral side of the stomach, opening by four interradial apertures between the oral arms, that is to say, by the four primitive apertures of the subgenital pits. In Nausithoe subgenital pits are absent altogether, and the same condition may be found in Charybdaeidae. The gastrovascular system shows every degree of complexity from a very primitive to a highly elaborate type of structure. Taking as a starting-point the wide archenteric cavity which the medusa inherits primitively from the antecedent actinula-stage (see article Medusa), we find, in such a form as Tessera, four interradial areas of concrescence between the exumbral and subumbral layers of endoderm, four so-called septal nodes or " cathammata," subdividing the stomach into four wide, radially situated pouches which communicate with each other beyond the septal nodes by wide apertures constituting what is termed by courtesy a ring-canal. In other cases the areas of concrescence may extend as far as the margin of the umbrella, so that the lobes of the stomach are completely separated from one From Bronn's Tierreich, ii. 2, "Coelenterata," by Carl Chun, by permission of C. F. Winter. FIG. I. - Haliclystus auricula. (After H. J. Clark.) Rudimentary tentacle of the tentaculocyst.
Glandular cushion.
Ocellus, and en, internal canal of the tentaculocyst.
Mouth.
Interradial septal ridges, passing into the taeniolae (f.t) in the stalk.
The eight adradial gonads on the subumbral walls of the four radial pouches, representing primitively four horse-shoeshaped gonads each divided into two by an interradial septum.
gen, another, as in Charybdaea (fig. 4), where there are four gastric pouches communicating with the central stomach by four so-called gastric ostia (fig. 4). A similar condition is seen in Pelagia, where the number of gastric pouches is increased to sixteen. In forms such as Lucernaria and Charybdaea, in which the umbrella is of deep form and the stomachcavity consequently of great extent in the vertical direction, the concrescence-areas or septal nodes are drawn out into vertical partitions or taeniolae (fig. 4, L.o.c.), resembling in their anatomical relations the mesenteries of the Anthopolyp. The phacellae are carried on the edges of the taeniolae (fig. 4, Gh). Finally in the majority of Scyphomedusae the primitively simple concrescence-areas become increased in number and in extent, so that radial canals, ring-canals, &c., can be distinguished in addition to stomachpouches. Thus in Aurelia (figs. 2a and 2b), to take a familiar example, the digestive tract begins with the mouth, of FIG. 2b. - Half of the lower surface which the four corners are of Aurelia aurita. The transparent prolonged into the four long tissues allow the enteric cavities and oral arms, perradial in position. canals to be seen through them. The mouth leads into the (From Gegenbaur.) spacious stomach containing a, Marginal lappets hiding tenthe four conspicuous horse taculocysts. shoe-shaped gonads (ov) mark, Oral arms. ing four stomach-pouches, b t tentacles. which, however, are inter- v, Axial or gastric portion of the radial in position. From the enteric cavity. stomach or its pouches arise gv, Radiating and anastomosing sixteen radial canals, four canals of the enteric system. perradial, four interradial and ov, Ovaries. The gastral filaments eight adradial (fig. 2b). The to these are not drawn. perradial and interradial canals near consist of a main stem giving off branches, and both stem and branches reach to the marginal ring-canal, the main stem ending in one of the eight tentaculocysts, which are lodged in the notches between the lobes of the umbrellar margin. The adradial canals are unbranched and run to the middle point of one of the marginal lobes. The system of canals shows great variation even in the same species.
The muscular system of the Scyphomedusae is developed on the subumbral surface as a system of circularly disposed fibres which by their contraction make the umbrella more concave and diminish its FIG. 3. - Scyphomedusae. a, Rhizostoma pulmo; b, Chrysaora hysoscella. cavity. The circular muscles usually form two chief portions, a peripheral wreath-muscle (Kranzmuskel), subdivided into four, eight or sixteen areas, and an oral ring-muscle round the mouth. Endodermal muscles are found in the phacellae, and in such forms as Lucernaria, longitudinal (vertical) muscular tracts or bands are found in the taeniolae, which, according to some authorities, are xxiv. 17 a of endodermal origin, but which, according to recent observations, are formed in the walls of the infundibular cavities, and are therefore of ectodermal origin.
The nervous system consists as in Hydromedusae of a diffuse plexus beneath the ectoderm, concentrated in certain places to form a central nervous system. In these medusae, however, the central nervous system does not form continuous rings, but occurs as four or eight separate con centrations at the margin of the umbrella, centred each round one of the sense-organs (tentaculocysts). Each nerve-centre controls its own antimere or segment of the body, receiving sensory impressions from the tentaculocyst and innervating its special subdivision of the muscular system. The separate nervecentres are, as a rule, placed in communication only by the general nerve-plexus, but in Charybdaea there is a zigzag marginal nerve connecting them up.
The sense-organs of the Scyphomedusae are on the whole of a very uniform type. They are always tentaculocysts, as already stated, and they always have a hollow axis, unlike the tentaculocysts of Hydromedusae, in which group these organs, when they do ,-?E(I Fr occur (as in Trachy linae) are always v?
solid. Two types of tentaculocyst must FIG. 4. - Charybdaea marsupialis. (After be distinguished, the Claus.) one occurring only in the order Stauromedusae, the other in all orders of the group. The second and commoner type is known as a rhopalium (fig. 6) and consists of a short, hollow rod, the wall of which is composed of the two bodylayers, ectoderm and endoderm, enclosing a cavity continuous with that of the gastrovascular system. At the apex of the rhopalium the endoderm is greatly thickened and consists of concrementcells secreting otoliths (Con). The more proximal portion of the rhopalium usually bears one or more ocelli (oc). The rhopalia are lodged in the notches be tween the marginal lobes of the umbrella, and each rhopalium is covered over by a little protecting flap or lappet. On the external (i.e. exumbral) face of the lappet there is frequently a patch of sensory ciliated epithelium regarded as olfactory in function and termed the olfactory pit (fig. 6, A). Each rhopalium is a centre round which, as already stated, nervous tissue is concentrated.
