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SCYPHOMEDUSAE
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SCYPHOMEDUSAE or ACALEPHAE, one of the two sub-divisions of the See also:Hydrozoa (q.v.), the other being the See also:Hydromedusae (q.v.). The subclass Scyphomedusae contains a number of animals which in the adult See also:condition are medusae or jelly-fishes (see See also:MEDUSA), exclusively marine in See also:habitat and found in all seas. They are chiefly pelagic organisms, floating at or near the See also:surface of the See also:water, but occur also at See also:great depths, and are sometimes fixed and sessile in See also:habit. Many See also:species attain a large See also:size and by their brilliant coloration are very conspicuous See also: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 See also:Maas (7 and 12). A scyphomedusa is distinguished from a hydromedusa chiefly by the following points. The See also:umbrella has a lobed, indented margin, a See also:character only seen amongst Hydromedusae in the See also:order Narcomedusae, and it is without the characteristic velum of the Hydromedusae; hence the Scyphomedusae are sometimes termed Hydrozoa Acraspeda. The sense-See also: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 See also:bear usually both ocelli and otocysts, and are hollow. The gonads are formed in the endoderm (hence " Entoearpeae "), and the generative products are See also:shed into the gastric cavity and pass to the exterior by way of the mouth. The development from the See also:egg may be See also:direct, or may take See also:place with an See also:alternation of generations (metagenesis), in which a non-sexual individual, the so-called scyphistoma or scyphopolyp, produces by budding the sexual medusae. See also:Morphology of the Scyphomedusa.—As already stated, a medusa of this order may be See also:free-See also:swimming or sessile in habit. Intermediate between these two types are species which have the See also:power of temporal fixation by the exumbralsurface. Such forms when undisturbed See also:fix themselves to the bottom and See also: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 See also:form is Lucernaria, See also:common on the See also:Atlantic coasts of See also:Europe, especially in Zostera-beds, attached to the See also:weed. It resembles in See also:general See also:appearance a See also:polyp, lacking even the characteristic medusan sense-organs, which are See also:present, however, in the allied genus Haliclystus (fig. I), proving its medusan nature beyond all doubt. The See also: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 See also:primitive 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 See also:long flexible filaments, hollow or solid, implanted singly on the margin of the umbrella (fig. 3, b), but in some species they occur in See also:groups or tufts (fig. 15), and in Lucernaria and its See also: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 See also:velarium (Ve), which is a flap See also:hanging down from the margin of the umbrella, and which consists of a See also: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 See also:simple structure at the extremity of the manubrium, or may be four-cornered, with the corners See also:drawn out into so-called oral arms, each of which bears on the inner See also:side a groove continuing the See also: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 See also:proboscis (fig. 3, a), in such a way that the mouth becomes nearly obliterated, and is reduced to a See also:system of See also:fine canals opening to the exterior by small pores. The mouth leads into the spacious See also:stomach, which is typically four lobed (fig. 2b, v). On the See also:floor of the stomach are See also:borne the conspicuous gonads (ov), and also tentacle-like processes termed gastric fila rnents 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 See also: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 See also:rule these medusae are of See also:separate sexes, but hermaphrodite forms are known, for example, the conspicuous See also:British (See also:east-Atlantic) medusa Chrysaora (fig. 3, b). Immediately below each gonad the subumbral ectoderm is pushed in, as it were, to form a See also:pit or deep cavity (fig. 2a, x, y) opening by a wide See also: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 vicinity water with its dissolved See also: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 See also:hernia, and hangs down in the subumbral space as if suspended by a mesentery (fig. 15). 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 See also: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-See also:stage (see See also: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 See also:courtesy a See also:ring-See also: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 II, From Broan's Tierreich, ii. 2, "Coelenterata," by Carl Chun, by permission of C. F. See also:Winter. of the Rudimentary tentacle tentaculocyst. Glandular See also:cushion. Ocellus, and en, See also:internal canal of the tentaculocyst. Mouth. Interradial septal ridges, passing into the taeniolae (f.t) in the stalk. gen, The eight adradial gonads on the subumbral walls of the four radial pouches, representing primitively four See also:horse-shoeshaped gonads each divided into two by an interradial septum. kl, oc, From the side. t', II. From above. A tentaculocyst (" colleto-cystophore" or " marginal See also:anchor ") seen from the subumbral side. Stalk. Subumbrella. Knobbed tentacles clusters. Tentaculocysts, four four interradial. IV. o, se, Su, in eight perradial, ra, another, as in Charybdaea' (fig. 4) , where there are four gastric pouches communicating with the central stomach by four so-called gastric See also:ostia (fig. 4). A similar condition is seen in See also: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 stomach-cavity consequently of great extent in the See also: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 See also:majority of Scyphomedusae the primitively simple See also:con-'2 crescence-areas become in-creased in number and in extent, so that radial canals, ring-canals, &c., can be distinguished in addition to stomach-pouches. Thus in See also:Aurelia (See also:figs. 2a and 2b), to take a See also:familiar example, the See also:digestive See also:tract begins with the mouth, of FIG. 2b.—See also:Half of the See also: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 See also:Gegenbaur.) spacious stomach containing a, Marginal lappets hiding ten- the four conspicuous horse- taculocysts. See also:shoe-shaped gonads (ov) See also:mark- b, Oral arms. See also:ing four stomach-pouches, 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 near to these are not drawn. perradial and interradial canals consist of a See also:main See also: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 See also:middle point of one of the marginal lobes. The system of canals shows great variation even in the same species. The See also:muscular system of the Scyphomedusae is See also:developed on the subumbral surface as a system of circularly disposed See also:fibres which by their contraction make the umbrella more See also:concave and diminish its hysoscella. cavity. The circular muscles usually form two See also:chief portions, a peripheral See also:wreath-muscle (Kranzmuskel), subdivided into four, eight or sixteen areas, and an oral ring-muscle See also:round the mouth. Endo-dermal muscles are found in the phacellae, and in such, forms as Lucernaria, See also:longitudinal (vertical) muscular tracts or bands are found in the taeniolae, which, according to some authorities, are of endodermal origin, but which, according to See also:recent observations, are formed in the walls of the infundibular cavities, and are therefore of ectodermal origin. The See also: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 See also:nerve-centre controls its own antimere or segment of the body, receiving sensory impressions from the tentaculocyst and innervating its See also:special subdivision of the muscular system. The separate nerve-centres 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 See also:uniform type. They are always tentaculocysts, as already stated, and they always have a hollow See also:axis, unlike the tentaculocysts of Hydromedusae, in which See also:group these organs, when they do - _, fi E17 occur (as in Trachy-L' ITCa linae) are always YC 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 See also:short, hollow See also:rod, the See also:wall of which is composed of the two body-layers, ectoderm and endoderm, enclosing a cavity continuous with that of the gastrovascular system. At the See also:apex of the rhopalium the endoderm is greatly Ge, thickened and con- sists of concrement- E U, cells secreting otoliths (Con). The more proximal See also:por- tion of the rhopalium usually bears one or Ve, more ocelli (oc). The Fr, rhopalia are lodged Tc in the notches be- EAx A, Natural size. B, View of the margin of the umbrella, natural size. C, See also:Horizontal See also:section through the umbrella and manubrium. D, Vertical sections, to the See also:left in the See also:plane of an interradius, to the right in the plane of a perradius. SU, Subumbrella. Ma, Manubrium. See also:EA x,Axial enteron. Gh and Fg, Gastral filaments (phacellae). CG, Corner groove. CR, Corner See also:ridge. SR, Side ridge. L.o.c., Endoderm lamella (See also:line of concrescence of the walls of the enteric cavity of the umbrella, whereby its single chamber is broken up into four pouches). Line of See also:attachment of a genital See also:band and band in section. Enteric pouch of the umbrella, in the left-See also:hand figure, points to the cavity uniting neighbouring pouches near the margin of the umbrella and giving origin to TCa, the tentacular canal. Velarium. Frenum of the velarium. Tentaculocyst. tween the marginal lobes of the umbrella, and each rhopalium is covered over by a little protecting flap or lappet. On the See also:external (i.e. exumbral) See also:face of the lappet there is frequently a patch of sensory ciliated epithelium regarded as olfactory in See also:function and termed the olfactory pit (fig. 6, A). Each rhopalium is a centre round which, as already stated, nervous See also:tissue is concentrated. The otoliths vary considerably in number and size. In Aurelia there are found numerous otoliths arranged irregularly. In Charybdaea (fig. 7, otol) the otoliths are larger but fewer in number and have a definite arrangement. In Nausithoe a single large otolith is found. The ocelli vary greatly both as regards number and complexity of eyes, with ectodermal pigment and See also:lens, are found also on the structure. In some genera they are absent, as, for instance, in Pelagic, rhopalia of Paraphyllina (Maas [8]). Cyanea and Rhizostoma. In Aurelia there are two on each rhopalium, The subumbral ocellus of Aurelia is found to be of the inverted a simple ocellus on the exumbral side, and a cupped ocellus on the subumbral side (not present in See also:young individuals). In Charybdaea there are no less than six ocelli on each of the four rhopalia (fig. y) ; on the exumbral aspect there are two median ocelli (oc', oc2), a distal and a proximal, each of them a vesiculate ocellus with a lens, and on the sides of the rhopalium are two pairs of ocelli without lenses (oc. l) ; some-times also an additional seventh ocellus occurs, a pit-like structure without a lens, either between the two median ocelli, or placed asymmetrically near the median proximal ocellus. (I) visual cells, sensory ectodermal cells, which may develop terminal visual cones; (2) pigment-cells, usually ectodermal, but in one known instance endodermal. The simplest type of ocellus is exemplified by the exumbral ocellus of Aurelia, a simple patch of pigment-cells interspersed with visual cells, the whole on a level with the remaining ectodermal epithelium. In the next stage of complication, seen in the supernumerary (seventh) ocellus of Charybdaea, the patch of pigmented and sensory epithelium is pushed in to form a little pit, in the H, See also:Bridge between the two marginal lappets forming the See also:hood. T, Tentaculocyst. End, Endoderm. Ent, Canal of the enteric system continued into the tentacu- locyst. [(auditory). Con, Endodermal See also:concretion oc, Ectodermal pigment (ocellus). The See also:drawing represents a section, taken in a radial vertical plane so as to pass through the long axis of the tentaculocyst. interior of which the pigment-cells secrete a gelatinous substance forming a rudimentary vitreous body. As a further advance, the pit becomes widened out into a See also:cup, as in the lateral ocelli of Charybdaea. The culminating stage of See also:evolution is seen in the median ocelli of Charybdaea (fig. 8); the primitively open cup has now closed over to form a vesicle lying beneath the ectoderm ; the See also:outer wall of the vesicle becomes thickened to form a cellular lens (1), while the proximal wall consists of sensory and pigmented cells and forms a retina. In this way the ocellus becomes a true See also:eye, very similar in See also:plan to the eyes of Gastropods and other molluscs. The ectoderm continued over the optic vesicle forms a transparent cornea (fig. 8, c) (better perhaps termed a conjunctiva), below which the spherical lens projects into the optic vesicle, imbedded in the vitreous See also:humour (v.b) which fills it; the retina (r) consists of visual cells with long cones (fig. 9) alternating with pigment-cells. The high development of- the eyes of Charybdaea is very remarkable, and so is their See also:close resemblance to the eyes found in other groups of the See also:animal See also:kingdom, with which they can have no genetic relation. Highly developed type, with the visual cones turned away from the See also:light, as in Tiaropsis amongst Hydromedusae, and here also the pigment is furnished by the endoderm, forming a cup into which the ectodermal visual cells project (Schewiakoff [131). In the Stauromedusae tentaculocysts are either absent altogether, as in Lucernaria, or represented by See also:peculiar structures termed " colletocystophores " or " marginal anchors " (fig. I, IV.). Each such body has a basal hollow portion (en) surmounted by a glandular cushion (hi), from the centre of which, projects a small, solid, See also:club-shaped See also:process or tentacle (t'). The basal portion bears an ocellus (oc) of simple structure. The distal club corresponds to the crystal-See also:sac of an See also:ordinary After Wladimir Schewiakoff, simplified from rhopalium, but bears a See also:battery a coloured See also:plate in Morphologisches 7ahrbuch, of nematocysts in place of the xv., 188g, by permission of Wilhelm See also:Engel- otoliths. These organs are See also:man- said to be used for purposes FIG. 7.—Tentaculocyst of Charybof adherence rather than to daea marsupialis, seen from the have the function of sense- right side. organs. st, Stalk. The histological structure oci, oc2, Distal and proximal median of the Scyphomedusae is in ocelli. the main similar to that of oc.l, Lateral ocelli. the Hydromedusae (q.v.), but otol, Otoliths (" crystal-sac "). the mesogloea is more abun- dantly developed in the free-swimming forms, and contains special mesogloeal corpuscles, derived by See also:immigration from the ectoderm, and generally occurring in the form of stellate or bipolar cells. Development of the Scyphomedusae.—No adult Scyphomedusae are known to reproduce themselves by budding or by any method other than the sexual one. The course of the development in this group is best made clear by taking as a type Aurelia, which, together with certain other common genera, such as Chrysaora and Cotylorhiza, has been studied in detail. Unfortunately the statements concerning some points are very contradictory. Combined from three figures by Wladimir Schewiakoff in Morphologisches Jahrbuch, xv., 1889, by permission of Wilhelm Engelman. The ova pass out of the mouth and are fertilized externally. In some cases the ova, after leaving the mouth, are lodged in the oral arms, and undergo the earliest phases of their development in this situation, accumulating in the grooves that continue the angles of the mouth, and bulging the wall of the groove into sacs or pockets. Ent oe ML ML Con 1, ?. (After Eimer.) In the left-hand figure ML, Marginal lappets. T, Tentaculocyst. A, See also:Superior or aboral olfactory pit. MT, Marginal tentacles of the disc. The view is from the aboral surface, magnified about 5o diameters. In the right-hand figure A, Superior or aboral olfactory pit. B, Inferior or adoral olfactory pit. The ovum undergoes See also:total cleavage, giving rise to a bastula which forms a gastrula (fig. to, A) by invagination (see article HYDROZOA). This is a type of germ-layer formation never found in the Hydromedusae, though of universal occurrence in all groups of animals above the See also:Coelentera. We may regard it as a form of unipolar immigration in which the immigrating cells pass into the interior in a connected See also:epithelial layer, instead of going in singly and independently. The embryo is set free as a planula larva (fig. to, B) in the gastrula stage, and the orifice of invagina tion or blastopore, which persists, is situated at the hinder See also:pole. After a See also:time the planula fixes itself by the anterior pole, with the blastopore uppermost. The larva after fixation changes into a polyp-like organism termed a scyphistoma or scyphopolyp (fig. to, C, D). The body becomes in shape like a See also:vase or See also:urn attached by a narrow stalk, round which a chitinous membrane is secreted. From the edges of the vase the four See also:primary tentacles grow out, each a slender filament with a solid endodermal axis. The tentacles border a broad, flattened peristome, from the centre of which arises the hypostome with the mouth at its extremity; the hypostome is at first See also:low, but soon becomes a projecting, See also:chimney-like See also:tube. It has been sought to prove that the interior of the hypostome is lined by ectoderm, so as to form a stomodaeum or ectodermal See also:oesophagus similar to that of the See also:Anthozoa, but this has been disproved by the most recent investigations of Hein (4) and Friedemann (3), who have shown that the mouth at the extremity of the hypostome represents the persistent blastopore of See also:nucleus; n.f, nerve the gastrula stage. fibril. The internal gastric cavity of the scyphistoma is not a simple space as in the hydropolyp, but is subdivided by four ridges or taeniolae, arising one in each interradius (fig. II, B). Each taeniola is similar in its anatomical relations to the similarly named structures in Haliclystus (fig. I), and becomes perforated in the same way ,at its outer side by a " septal ostium," forming as it were the rudiment of a ring-canal. Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a See also:funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub-genital cavities of other Scyphomedusae. It is asserted, however, by Friedemann (3), a recent investigator of the subject, that the infundibular cavities appear See also:late in the scyphistoma and have no relation either to the septal muscles or to the subgenital cavities of the adult. The muscle-bands are very contractile, rendering the scyphistoma one of the most difficult of all organisms to preserve in an See also:expanded condition. By their contraction the muscles of the taeniolae See also:drag the hypostome down and so produce the appearances which have been interpreted as a stomodaeal invagination. As the scyphistoma grows the tentacles increase in number, four interradial and eight adradial being formed in addition to the four primary perradial tentacles (fig. A, B, C). The animal may produce its like by lateral budding, or by budding from a basal stolon. The scyphistoma of IVausithoe forms a branching network which grows in the sponge Esperella and forms the colonial polypoid organism named by Schulze Spongicola fistularis, by See also:Allman Ste phanoscyphus mirabilis. Sooner or later, however, the scyphistoma produces free medusae by a process of transverse fission termed strobilization. In the simplest See also:case one medusa, or at least one at atime, is produced in this way (monodisk strobilization) ; a circular furrow cuts off the upper, tentacle-bearing portion from the lower half of the scyphistoma (fig. ii, D, and fig. 12), and the upper See also:part becomes detached and swims away, while the See also:base. regenerates a new See also:crown. In most cases, however, many such furrows are formed (polydisk strobilization), so that the animal comes to resemble a See also:pile of saucers one above the other (fig. 12). The uppermost saucers of the pile become detached uccessively and swim off. In this See also:state the scyphistoma is termed a strobila. The medusae produced by strobilization of the scyphistoma are of a peculiar type termed Ephyrae (fig. ii, E, F). As preparations P P P A~ Ad In D P E (After Claus.) A, Scyphistoma of Chrysaora, with four perradial tentacles and horny basal perisarc. B, Oral surface of later stage of scyphistoma of Aurelia, with commencement of four interradial tentacles. The quadrangular mouth is seen in the centre; the outline of the stomach wall, seen by transparency around it, is nipped in four places interradially to form the four gastric ridges. C, Oral surface of a sixteen-tentacled scyphistoma of Aurelia. The four gastric interradial ridges are seen through the mouth. D, First constriction of the Aurelia scyphistoma to form the pile of ephyrae or young medusae. The single ephyra carries the sixteen scyphistoma tentacles, which will See also:atrophy and disappear. The four longitudinal gastric ridges are seen by transparency. for their formation the margin of the peristome of the scyphistoma grows out into eight lobes, four perradial, four interradial.. The sixteen tentacles of the scyphistoma disappear, and in the place of the four perradial and four interradial tentacles, the eight tentaculocysts of the adult are formed as outgrowths of the subumbral margin, independently of the tentacles of the scyphistoma (Frieder See also:mann). The septal ostia become widened and the gastral cavity flattened, whereby the taeniolae become comparatively shallow columns, similar to the septal nodes or cathammata of other forms? The ephyra has a See also:flat, disk-shaped body, with eight marginal lobes (four perradial, four interradial) ; a tentaculocyst is lodged in a deep notch at the apex of each See also:lobe. Four groups of phacellae indicate the four interradii. The stomach has sixteen marginal pouches and the general anatomical structure recalls that of Pelagia. As the i The four primitive interradial cathammata disappear in the fully formed ephyra and become replaced by sixteen subradial concrescence-areas without any ostia or ring-canal at the margin. ,n After W. Schewiakoff, simplified from a coloured plate in bforphelogisches Jahrbuch, ay., 1889, by permission of Wilhelm Engelman. A, Diblastula stage. B, Stage after See also:closure of blastopore. C, Fixed larva. D, Later stage with mouth, short tentacles, &c. ep, Ectoderm. hy, Endoderm. pe, Stomodaeum. m, Mouth. bl, Blastopore. E, Young ephyra just liberated, showing the eight bifurcate arms of the disk and the interradial single gastral filaments. F, Ephyra developing into a medusa by the growth of the adradial regions. The gastral filaments have in-creased to three in each of the four sets. A, Margin of the mouth. Ad, Adradial See also:radius. F, Gastral filament. In, Interradial radius. JG, Adradial gastral canal. JR = R3, Adradial lobe of the disk. K, Lappet of a perradial See also:arm. M, Stomach wall. Mst, Muscle of the gastral ridge. Mw, Gastral ridge.. Ms, Mesogloea. 0, Tentaculocyst. P, Perradial radius. See also:R2, Interradial radius. R3, Adradial radius. SG, Commencement of lateral See also:vessel. ephyra grows in size it gradually takes on the form and structure of the young medusa. The adradial regions grow (fig. F) so as to See also:change the See also:star-like See also:contour into one more evenly circular, the tentacles grow out, and the various parts become complicated and take on the structure of the adult medusa. The course of development sketched out above is that which is typical of the higher forms of Scyphomedusae, and is by no means to be regarded as the most primitive type of development. The complicated alternation of generations seen in such a form as Aurelia does not occur in the more primitive genera. Thus in Pelagia the scyphistoma-stage is free-swimming and changes directly into the ephyra, which in its turn grows into the adult form. On the other hand, such a form as Lucernaria or Haliclystus may be regarded simply as a scyphistoma which has become adult and mature. The comparison of the metagenetic type of development, such as that of Aurelia, with the more primitive genera of Scyphomedusae, indicates clearly that the scyphistoma and ephyra are recapitulative larval stages which are represented by the adult forms of primitive genera, making such allowances as are necessary when comparing adult and larval forms. The metagenesis has arisen through the scyphistoma-larva acquiring the power of larval proliferation by budding. A similar origin for metagenesis has been discussed under the Hydromedusae (q.v.). The above comparison further indicates that the scyphistoma should not be regarded as a polyp but rather as a medusoid organism. The only certain criterion of a medusa-individual is the presence of definite sense-organs, but in cases where the organism is much reduced, this criterion may fail us, as it does in the genus Lucernaria. Nevertheless a comparison between Lucernaria and its close ally Haliclystus shows clearly that the See also:absence of sense-organs in the former is the result of secondary reduction, so that a true medusa may lose its most characteristic feature. Hence the absence of sense-organs in the scyphistoma does not necessarily disprove its medusoid character, while its anatomical structure resembles that of a simple scyphomedusa, such as Lucernaria, rather than that of a polyp. See also:Affinities of the Scyphomedusae.—By some authorities the Scyphomedusae have been removed from the Hydrozoa and See also:united with the Anthozoa in a common group termed Scyphozoa. The diagnostic features of the class Scyphozoa thus constituted are supposed to be (1) an ectodermal oesophagus or stomodaeum, (2) a gastric cavity subdivided by mesenteries, (3) gonads formed in the endoderm. It appears, however, that the first of these characters is non-existent, and that the so-called mesenteries are simply the concrescence-areas found in all medusae. There remains only the third feature, the endodermal gonads, as an See also:argument for uniting the Scyphomedusae with the Anthozoa, against which must be set all the peculiarities of medusan organization in which the Scyphomedusae resemble the Hydromedusae. The fact that the Scyphomedusae have a number of well-marked peculiarities of form and structure is not incompatible with placing them in the Hydrozoa as a distinct sub-class, contrasting sharply in many ways with the Hydromedusae. Additional information and CommentsThere are no comments yet for this article.
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