Cnidarians exhibit primary radial symmetry. In the Hydrozoa and Scyphozoa all diameters are apolar (i.e. have like ends). Furthermore, any two diameters at right angles to each other are also alike, dividing the animal into symmetric halves. The Anthozoa are either biradial or bilateral. Biradial Anthozoa have apolar diameters with the sagittal axis aligned along the long axis of the mouth and defines two symmetric halves that differ distinctively from the transverse axis, which is at 90o to the sagittal. In bilaterally symmetric Anthozoa the sagittal axis is heteropolar (i.e. it has unlike ends) and so the animal has distinct dorsal and ventral surfaces and the only exact plane of symmetry is the sagittal plane.
The cnidarian polyp consists of a base, a stalk and the oral end. The base has a rounded or pointed end thrust into the substrate or alternatively an adhesive pedal disc, or an adhesive skeletal secretion or root-like stolons. The column, stem or stalk forms a cylinder. The oral end of the hydrozoan polyp is an elongated vase-shaped hydranth bearing a circular mouth. In the Anthozoa the oral end is an expanded oral disc and the mouth is often elongated with a ciliated groove (siphonoglyph) at one or both ends.
The oral end bears tentacles, though tentacles are absent in minute polyps, such as Microhydra and Protohydra, and are also absent in some parasitic hydroids, e.g. Hydrichthys, and also in rhizostome medusae, the anemone Limnactinia and some modified morphs in polymorphic species. The tentacles are irregularly scattered or more usually arranged in one or more circlets. The tentacles may be hollow or solid and are armed with nematocysts.
The polyp wall consists of epidermal and gastrodermal epithelia with mesogloea inbetween the two epithelia. The mesogloea is a thin sheet of cement, constituting a mesenchymal or fibrous connective tissue.
Hydroid polyps possess a tubular coelenteron partially divided by 4 septa. In Anthozoa the pharynx comprises a stout tube hanging down from the mouth into the coelenteron and in some anthozoans septa attach the pharynx to the body wall.
There are various types of modified polyp in polymorphic species. Tentaculozooids have no mouth and no coelenteron and have a tentacular function. Gonozooids have no tentacles and no digestive tract and have a reproductive function. Siphonozooids have an emphasis on current-producing devices.
Gonophores bud from stolons, hydranths, the hydrocaulus or from modified hydranths (gonozooids or blastostyles) or often in a definite sequence borne on the gastrozooids, with the oldest situated most basally. Gonozooids form from gastrozooids by reduction and loss of the tentacles, closure of the mouth, and reduction of the gastrovascular cavity. Gonophores may have a periderm covering, called the gonotheca.
A gonangium consists of blastostyle covered by gonotheca and one to many gonophores. The oldest gonophore bud occurring near to the apex. The gonotheca may be smooth, spined or ribbed. A gonangium with or without protective modifications, e.g. phylactocarps, is a modified hydrocladium (branch) or its accessory branches.
The medusa is the active locomotory stage and the sexually mature form.
The medusa body consists of a deep to shallow bowl of gelatinous nature, the bell or umbrella. The convex aboral surface is the exumbrella and the concave oral surface is the subumbrella. Sense organs and tentacles are borne on the umbrella rim. The mouth hangs down from the centre of the subumbrella on the end of a tubular (endoderm-lined) projection called the manubrium, which leads into the gastric cavity or stomach in the bell centre. In Hydromedusae the stomach is usually a simple chamber. In Scyphomedusae the stomach periphery may be divided into 4 perradial gastric pouches by 4 interradial septa. The stomach gives off the gastrodermal canals: four, or a multiple of four, radial canals and the ring or circular canal in the bell margin.
The medusa has tetramerous symmetry and the radial canals, tentacles and sense organs are all arranged tetramerously. There are four perradii at right angles to each other, and in line with the four radial canals. The interradii comprise the sectors between the perradii. The midradius of an interradius is an adradius.
Ectodermal epithelium lines both outer surfaces of the bell. Endodermal epithelium lines the manubrium and the radial and ring canals. There is a thick layer of jelly (mesogloea or collenchyme) between the ecto- and endodermal epithelia. Hydromedusae possess a velum: a circular shelf projecting inward from the bell margin and partially cutting off the subumbrellar space. Medusae possessing a velum are called veiled or craspedote medusae. Scyphomedusae have no velum and are acraspedote. The velum functions with the circular muscle band in swimming.
Some have undergone loss of tentacles, mouth and reproductive system to form a locomotory bell or float for the polyp colony. Others have lost their non-reproductive structures and have no independent life.
Cnidaria are at the tissue grade of construction and the sense organs of the medusae are the only organs proper.
The epidermis (ectoderm) is cellular or syncytial. When cellular the cells may be cuboidal to columnar or flat (as on the exumbrellar) or slender and elongated (as in anemones). A cuticle covering may be present, or alternatively the epidermis may be ciliated (as in anemones) or flagellated (as in some medusae). The epidermal cell cytoplasm consists of strands with fluid-filled spaces between. The bases of the epidermal cells are fastened to the mesogloea by pseudopodial processes. On exposed parts of hydroid polyps and medusae (except the exumbrella) the cell bases give off two long strands that run longitudinally and external to or else embedded in the mesogloea. These processes contain a contractile fibre (myoneme) and the cells are therefore epitheliomuscular cells. In hydras each epidermal cell gives out two opposite basal strands. In some Trachylina, most Scyphozoa and in Anthozoa the amoeboid epidermal cell bases lack contractile extensions.
Nematocysts and cnidoblasts occur on the epidermis of the tentacles, oral regions, and also sometimes on the stems of polyps and sometimes on the exumbrella of medusae. The nematocysts are often in wart-like clusters or ascending tracts.
Epidermal gland cells occur on the tentacles, oral regions, pedal disc, on the pharynx of anthozoans and on exposed parts of the column. The pedal disc epidermis of hydra consists entirely of gland cells with muscular basal extensions (glandulomuscular cells). Mucous cells provide attachment, protection and are involved in entanglement of prey and debris and provide lubrication to assist swallowing of food. Granular cells are common in the pharyngeal lining and are sparse on the surface epidermis.
In Trachylina, Scyphozoa and Anthozoa the epidermal muscle fibres are usually independent fibres sunk into a subepidermal position. These may form a layer or may form bundles close to or embedded in the mesogloea. The muscle bundles may have a mesogloeal core. Epidermal / subepidermal muscle fibres may be fastened to the surface of scallops or folds of the mesogloea, an arrangement which serves to increase the number of fibres. In polypoids the muscle is of the smooth type, whilst the circular muscle fibres of the velum and subumbrella in medusae are cross-striated.
Sensory cells are common in the epidermis of the tentacles and oral regions, and may occur singly or in patches (often in medusae). Each sensory cell terminates in one or more bristles or a bulb or a long motile flagellum and gives out a projection to the nerve plexus.
Interstitial (indifferent) cells are undifferentiated cells scattered among the epidermal supporting cells. These cells can differentiate into nematocysts or sex cells or into other cell types during regeneration. The body wall thus consists of three strata: the outer supporting cells and glandular and sensory cells, the nervous stratum and the muscle stratum. Beneath these is the mesogloea.
The gastrodermis is an endoderm of cuboidal to columnar nutritive cells and interspersed glandular and sensory cells. Some of the nutritive cells have myonemal basal extensions and are nutritive-muscular cells. The myoneme extensions form circular muscle in polypoids. Medusae usually lack gastrodermal muscle extensions or a gastrodermal muscle layer. Circular and longitudinal gastrodermal muscle occurs in anthozoans and these are more strongly developed than the epidermal muscle and may be separate from the gastrodermis. In Anthozoa the gastrodermal muscle attaches to mesogloeal folds or plates. The gastrodermis is columnar and often folded where digestion occurs and contains food vacuoles and each nutritive cell usually possesses two flagella.
Mucous gland cells are very abundant near the mouth and are flagellated and have muscular basal extensions. Granular gland cells occur in the gastrodermis and are possibly involved in enzyme secretion (?). These granular cells may be flagellated and their bases may or may not reach the mesogloea.
Nematocysts occur on certain gastrovascular structures in Scyphozoa and Anthozoa, such as the gastric filaments, septal filaments and acontia and are otherwise absent from the gastrodermis. The subgastral nerve net is less developed than the epidermal one.
Zoochlorellae are found in the gastrodermis of the green hydra and yellow zooxanthellae in corals and anemones.
The mesogloea is the layer between epidermis and gastrodermis. In hydrozoan polyps the mesogloea is a thin gelatinous membrane devoid of cells and fibres, forming a mesolamella. In medusae the mesogloea is gelatin-like and constitutes the bulk of the animal. In hydromedusae it contains few cells and fibres of unknown origin (?) and is a mesogloea proper. In scyphomedusae the mesogloea contains fibres and amoeboid cells and constitutes a collenchyme. All medusae also possess a mesolamella between the mesogloea and the epi- and gastrodermises. In medusae (Aequorea, Rhizostoma) the mesogloea contains no gelatin or mucin and so is not connective tissue of classical nature. Anthozoa contain a cellular mesogloea or mesenchyme. This consists of stellate amoeboid cells in a jelly matrix or in a fibrous connective tissue, often comprising several layers of different orientation.
Sensory cells (palpocils) are common in the epidermis of tentacles and oral regions. They occur singly, or, as is often the case in medusae, they occur in patches. Each sensory cell terminates in one or more bristles or a bulb or a long motile flagellum. A basal process connects each sensory cell to the nerve plexus.
Interstitial (indifferent) cells are scattered among the epidermal supporting cells. These are undifferentiated cells that may differentiate into nematocysts, sex cells or other cells during regeneration. Thus, the covering layer comprises three strata: the outer stratum of supporting, glandular and sensory cells, the nervous stratum and the muscle stratum, backed by mesogloea.
The gastrodermis (endoderm) consists of cuboidal-columnar nutritive cells, some of which have myoneme basal extensions and are therefore nutritive-muscular cells. The myoneme extensions are circular in polypoids, circular and longitudinal in Anthozoa, but are usually absent from medusae. In Anthozoa the gastrodermal musculature is more strongly developed than the epidermal musculature and may be separate from the gastrodermis and adherent to mesogloeal folds or plates. Glandular and sensory cells also occur in the gastrodermis.
Medusae are 95-96% water (increasing to 98% in brackish water) and less than 1% organic material, the rest being salts. The organic material is mostly a low nitrogen protein, possibly chitin (?).
The gut constitutes an epithelial sac that may be subdivided, by septa in polyps, or into a central stomach and radial and marginal canals in medusae. In scyphozoa the gastric filaments contain nematocysts and are borne on septa. In Anthozoa the free edges of the septa form a sinuous cord (septal filament) containing gland cells and nematocysts. The digestive tract is continuous between individuals in colonial forms.
Cnidarians are carnivorous and feed on animals and bits of animals, etc. The tentacles bear nematocysts and produce adhesive secretions to immobilise the prey. The tentacle(s) bends towards the mouth, the mouth opens and the food is grasped by the mouth rim. Food is swallowed, aided by mucous secretion from the gastrodermis of the manubrium or pharynx, and sometimes by ciliary action, and sometimes by muscular action. The body is highly distensible and cnidarians can swallow large objects. Some anthozoans feed by a mucous-ciliary method in which mucous strands entangle particles that are then conveyed to the mouth by ciliary action.
Digestion is extracellular by enzyme secretion and intracellular by phagocytosis. Alkali proteolytic secretions are produced by the gastrodermis (and septal filaments in Anthozoa) and is rapid, taking 8-12 hours. The resultant particles are phagocytosed into food vacuoles inside the gastrodermal cells, in which the secretion changes from acid to alkaline. Most cannot digest starch inside these vacuoles. Intracellular digestion is slower, taking several days. Waste is ejected through the mouth. Excess nutrients are stored in the gastrodermis as fat and glycogen.
In polyps the muscular system consists of an outer cylinder of (irregularly spaced) longitudinal fibres formed from the base of the epidermis and an inner cylinder of circular fibres comprising the base of the gastrodermis. The longitudinal fibres bend the body and tentacles and contract the animal. The circular fibres extend the animal and tentacles and produce peristaltic waves employed in swallowing food and in locomotion, etc.
In medusae the gastrodermal muscle is nearly or totally absent and the epidermal cylinder is reduced to longitudinal fibres in the tentacles and manubrium, and to radial fibres in the subumbrella, and to circular epidermal muscle cells in the velum and subumbrella.
In Anthozoa the epidermal muscle is reduced to fibres in the tentacles and oral disc. Longitudinal bands strengthen the anthozoan gastrodermal layer. In Scyphozoa and Anthozoa, rather than forming cylinders, the muscles may be festooned on mesogloeal plates that penetrate the epithelia, or the muscles may be sunk into the mesogloea as bundles.
In pedal and oral discs the epidermal fibres are radial, whilst the gastrodermal fibres are circular.
There is no specialised respiratory mechanism. The thin body wall facilitates diffusion and the tentacles increase surface area for exchange. In the gastrodermal cilia/flagella drive currents through the gastrodermal system. Body movements also mix the gastrovascular contents. In medusae there are flagellar currents in the canal system and in Anthozoa the ciliated siphonoglyph generates a water stream. Scyphozoa possess four deep pits in the subumbrella, of unknown function, that are possibly respiratory.
Respiration is aerobic, but the rate is very low in medusae because of their very small organic content. In Anthozoa respiration is low when the animal is contracted and increases when the animal is extended, and requires clean aerated water.
There are no special excretory structures. In some hydromedusae and scyphomedusae, each radial canal opens near to the tentacles via a pore that has been seen to eject non-nutritive particles and possibly has an excretory function. The gastrodermis may therefore have an excretory function.
In some Siphonophora there is a mass permeated by gastrodermal canals beneath the float. This mass possibly has an excretory function (?) and contains guanine crystals.
In the Anthozoa the septal filaments and septa are excretory and contain xanthine crystals. In Alcyonium and some hydroids amoeboid cells transport waste to the exterior.
Urea, uric acid, creatine and creatinine are absent. Xanthine and guanine are found in the gastrodermis and may be waste products. Anemones and medusae excrete ammonia. In anemones ammonia accounts for 77-100% of the nitrogenous waste, and there are also traces of urea and uric acid.
Sensory nerve cells in the epithelia connect to the subepithelial plexus, outside the muscle layer. This plexus may be connected to the less-developed sub-gastrodermal plexus, which is known to exist in some Cnidaria and possibly exists in all cnidarians. Nerve cells exist in the mesogloea and may serve to connect the two plexi.
In hydromedusae the plexus is limited to the subumbrellar surface and connects with two nerve rings in the bell margin. The upper marginal nerve ring communicates with the marginal sense organs, whilst the lower ring controls the velum ring muscle. In scyphozoans there are no marginal nerve rings. Ganglia near each rhopalium connect to the subumbrellar plexus.
In general there is a diffuse radial nervous system, which is synaptic, and conducts equally well in all directions. In anemones stimuli conduct longitudinally more readily than transversely. Cnidarians are characterised by autonomy, or the independence of parts. Isolated portions maintain their behaviour. Isolated tentacles and acontia move as normal when stimulated and pedal discs creep about. There is little or no development of a CNS. Characteristic reflexes include contraction of anemones, righting of medusae and opening of the mouth and pharynx when the tentacles are stimulated by food. Generally, however, there are few reflexes.
Cnidarians characteristically possess independent effectors. Nematocysts, gland cells and cilia react independently of the nervous system. It is thought that the longitudinal muscles of anemone acontia also react independently of the nervous system. The nerve plexus conducts without decrement or fatigue at signal velocities of 7-120 cms-1 (cf. 12 500 cms-1 in humans).
Class Scyphozoa (Scyphomedusae)
The scyphozoa are acraspedote, tetramerous medusae or medusa-like polypoids. The gastrovascular cavity bears endodermal tentacles (gastric filaments) and gonads and is divided in the adult or larva by four interradial septa into a central space and four perradial pockets. The subumbrella is indented by four interradial pits (subumbrellar funnels or peristomial pits) sunk into the septa.
The mesogloea is cellular. The marginal bodies are reduced tentacles or tentaculocysts (rhopalia) with endodermal statoliths. The polypoid larva (scyphistoma) is tetramerous with four longitudinal septa dividing the gastrovascular cavity and either transforms directly into the adult or gives off medusae by transverse fission.
Scyphozoans exhibit tetramerous radial symmetry. The oral-aboral axis is at the intersection of the two principal planes of symmetry, which intersect at right angles. Gastric pockets and mouth arms or mouth angles lie on the perradii. The interradii are halfway between the perradii and locate the four septa and subumbrellar funnels. The four marginal bodies are situated on the perradii in Cubomedusae or on the interradii in Coronatae. When eight in number, the marginal bodies occur on both the perradii and the interradii. When more than eight marginal bodies are present they also occur on the adradii, which are halfway between each per- and interradius. Parts usually occur in fours or multiples of four. Some scyphozoans, however, are based on a plan of six. The tentacles may be indefinite in number.
The bell is goblet- or trumpet-shaped in the Stauromedusae, cuboidal or pyramidal in the Cubomedusae and dome-, bowl-, or saucer-shaped in the other orders. In the Coronatae the exumbrellar surface bears a horizontal circular groove, the coronal groove. The bell is usually very gelatinous and often firm or cartilage-like. Jelly is situated on the exumbrellar and subumbrellar side of the gastrovascular system and extends into the tentacles and oral arms. There is no true velum, though there is a subumbrellar extension, termed the velarium, in Cubomedusae and in Aurelia.
Both surfaces of the bell may bear nematocysts, which are either strewn or else grouped into warts or clusters. The bell margin is usually scalloped into lappets and bears tentacles and sensory bodies (usually x 4 or a multiple of four). These appendages are symmetrically arranged: the sensory bodies reside in niches between the lappets, whilst the tentacles occur between the sensory bodies, in niches or on lappets or on the subumbrellar surface. The tentacles are usually definite in number, but are absent in the Rhizostomeae. The tentacles are borne on gelatinous basal expansions (pedalia) in the Cubomedusae and Coronatae. Tentacles are solid in the Coronatae, but otherwise hollow, and are very motile and contractile. In the Stauromedusae the tentacles are capitate, that is they bear nematocyst-filled heads, but are filiform in other groups with nematocysts either strewn throughout or restricted to one surface or in rings, warts, etc.
Four interradial subumbrellar funnels (peristomial pits) occur in the orders Stauromedusae, Cubomedusae and Coronatae. These have an unknown function (?) and possibly aid in respiration (?) since water passes in and out of them as the bell contracts. In the Semaeostomes and Rhizostomeae the subumbrellar funnels only occur in the larvae, and in their place the adults possess shallow depressions or subgenital pits.
The manubrium depends from the centre of the subumbrellar and bears the four-cornered mouth on its terminus. The angles of the mouth are often drawn out into four perradial lobes, either short or extended into long frilly oral arms, as in the Semaeostomeaea and the Rhizostomeae. In the Rhizostomeae the oral arms are branched and fuse at their edges to obliterate the mouth and instead bear 100’s to 1000’s of minute ‘suctorial’ mouths. The frilled edges of the oral lobes or arms bear batteries of nematocysts and may grow out into a fringe of nematocyst-bearing tentacle-like projections.
The quadrangular manubrium (also called gullet or pharynx) is lined by endoderm, hence there is no stomodaeum in Scyphozoa. The manubrium connects the mouth to the gastrovascular cavity. From the wall of the exumbrella project four septa (comprised of gastrodermis, epidermis and mesogloea) along the interradii, partway into the interior. This arrangement results in an undivided central stomach and four perradial gastric pouches/pockets that collectively form the coronal/coronary stomach. Each septum is pierced peripherally by a circular opening, the septal ostium, resulting in a ring sinus. The subumbrellar funnels penetrate deeply into the mesogloea of the septa (causing each septum to bulge laterally and narrowing the passages between the central and coronal stomachs.
The septal muscle is a strong longitudinal band (of ectodermal origin) in each septum. The free inner edge of each septum bears numerous tentacle-like gastric filaments (digitelli) arranged in a row (phacella) on each side. Each has a mesogloeal core with a gastrodermis covering possessing nematocysts and gland cells.
The arrangement of the gastrovascular system so far described pertains to adults of the Stauromedusae, Cubomedusae and Coronatae, but is restricted to the scyphistoma of other orders. During transverse fission of the scyphistoma, the septa degenerate and the larval funnels flatten out and subgenital pits arise in their place. There is a large central stomach whose margin may be slightly scalloped into four or more divisions, but there are no gastric pockets. Numerous gastric filaments spring from the stomach floor in interradial groups. From the periphery of the stomach spring simple, branched or anastomising radial canals or wide channels that may give rise to a ring canal at the periphery. The gonads are bands or sacs in the stomach floor and may hang down into the subumbrellar cavity.
The Stauromedusae possess two types of nematocyst: atrichous isorhizas and microbasic heterotrichous euryteles. These two types also occur in the semaeostomes and rhizostomes, but the adults of these orders may also have a third type: holotrichous isorhizas. (Nematocyst types of other orders?)
The mesogloea consists of jelly, fibres that stiffen the bell and loose amoeboid cells (of epidermal origin?) and hence is a collenchyme and is situated between the epidermis and the gastrodermis. The mesogloea may have a cartilaginous consistency. The mesogloea is separated from the epidermis and the gastrodermis by a mesolamella.
The epidermis consists of flattened to columnar cells, and may be flagellated on the subumbrella and elsewhere and possesses nematocysts, sensory cells and mucus-secreting gland cells. In the Stauromedusae, the anchors and pedal disc contain special gland cells, as does the pedal disc of the scyphistoma. Sensory epithelium occurs in the rhopalia.
The coronal muscles, and sometimes also the longitudinal tentacle muscle fibres, are cross-striated. Other muscle fibres are of the smooth muscle type. In Pelagia, the longitudinal tentacle muscle fibres are independent of the epidermis and are attached to lengthwise folds of the mesolamella.
The gastrodermis may be regionated and is flagellated. The flagellation gives rise to definite currents through the gastrovascular system.
The muscular system is mostly epidermal and restricted to the margin and subumbrellar. The coronal muscle is a strong, broad, circular muscle band in the subumbrella and is often divided into 4, 8 or 16 sections by radial mesogloeal partitions to give it a polygonal shape. The coronal muscle is the powerful swimming muscle and is reduced to a narrow marginal band in the sessile Stauromedusae).
Longitudinal epidermal muscle fibres form a layer in the manubrium and oral lobes and strong longitudinal epidermal fibres occur in the tentacles and pedalia and in the axes of the long oral arms of Semaeostomes. They also form a radial sheet or (usually 4, 8 or 16) radial bands in the subumbrella, extending from the manubrial base to the inner edge of the coronal muscle or through the coronal muscle along the mesogloeal partitions. In some forms the radial bands fan out towards the manubrium, forming the delta muscles. The lappets may contain circular muscle fibres that are extensions of the coronal muscle, or short radial fibres. When present, the septa contain strong longitudinal bands that contract the animal; these are best developed in the Stauromedusae, but also occur in the scyphistoma and some other orders. Circular gastrodermal muscles are apparently absent in Scyphozoa (?).
There is no marginal nerve ring except in Cubomedusae. The rhopalial ganglia (innervating the sensory niches or rhopalia) connect to the subumbrellar plexus, which is concentrated into radial strands along the main radii. Rhizostoma has an approximately circular arrangement of the main part of the subumbrellar plexus. The rhopalial ganglia are only directly connected to each other in Cubomedusae. The subepidermal plexus innervates the manubrium, oral lobes and tentacles. The subgastral plexus innervates the walls of the gastrovascular system in Stauromedusae (other scyphozoans?). Nervous tissue also accompanies the nematocyst batteries.
The marginal sensory bodies occur as rhopalioids in the Stauromedusae and as rhopalia in the other orders. Rhopalia occur on the sides of the bell in Cubomedusae, on the pedalia in most Coronatae and in niches between the marginal lappets in other Coronatae, Semaeostomeae and Rhizostomeae. Each rhopalium is housed in a sensory niche in which a hood-like exumbrella extension forms the roof. The Semaeostomae and Rhizostomae have rhopalia either side of rhopalial lappets. These rhopalial lappets develop from the ephyra larval lappets that may remain as protective lappets for the rhopalia.
Each rhopalium develops at the base of a primary larval tentacle as a small hollow club or tentaculocyst (cf. Lithostyles of Trachylina). The free end of the rhopalium has an interior gastrodermal lining that forms a heaped mass of polygonal cells, each of which contains a statolith, which with overlying epiderm, forms a statocyst, an organ of equilibrium. The statoliths are calcium sulphate (gypsum) with some calcium phosphate. The epidermis at the sides and base of the rhopalium is thickened into a sensory epithelium. This sensory epithelium possesses long sensory hairs over most or over certain parts of its surface and is underlain by nerve plexus.
In most Semaeostomeae and Rhizostomeae the exumbrellar side of the hood over the sensory niche possesses a sensory pit with an epithelium lining, called the outer olfactory pit (olfactory?) and a similar inner olfactory pit may occur in the floor of the sensory niche.
Scyphozoan medusae can perceive light and avoid bright sunlight, descend at midday and in darkness and surface in the morning or late afternoon and also surface during cloudy days. The medusae are inactive in strong light and darkness, and active in diffuse light. Others, however, prefer sunlight. Medusae descend in rough or stormy weather. Aurelia has eyespots (rhopalial ocelli) and light increases its bell pulsation rate. Cyanea has no eyespots and light has no effect on pulsation rate. Medusae without eyespots may still perceive light (possibly via the general sensory epithelium?). The exumbrella is apparently devoid of sensory perception.
Some Scyphozoa, for example the Cubomedusae, some Coronatae and Aurelia, possess ocelli on their rhopalia, either pigment-spot ocelli, pigment-cup ocelli or complicated eyes each with a lens. Semaeostomes and rhizostomes generally lack ocelli.
Scyphozoans are carnivorous. The tentacles catch food and deliver it to the mouth. The gastric filaments and gastrodermis of the septa secrete an acid digestive fluid that reduces food to a broth in about six hours. Particles are then phagocytosed by the gastrodermal cells within which they undergo intracellular digestion. The food vacuoles are passed to the mesogloea, where amoebocytes and epidermal cells take them up. The gastrodermis of the gastric pouches stores protein, fat and glycogen. The gastric filaments are not involved in absorption or food storage.
Ephyrae eat mostly protozoans, but Aurelia ephyrae have been seen to also eat hydromedusae and ctenophores. Food is caught by the lappets and entangled in mucus. The lappets curve toward the mouth and wipe the food onto the manubrial edge. Flagella currents on the exumbrella carry small food particles from the centre to the periphery and hence to the lappets and then subumbrellar currents pass food particles on to the margin and then to the manubrium.
Adult semaeostomes catch food with their tentacles and / or oral arms. Aurelia eats mostly planktonic molluscs, crustaceans, tunicate larvae, copepods, rotifers, nematodes, young polychaetes, protozoans, diatoms and eggs. These collect primarily on the exumbrellar surface where they become entangled in mucus and are carried to the margins by ciliary/flagellary currents. At the margins they collect as eight masses in the centre of the lappets and are licked off by the frilled edges of the oral arms and then carried by flagellary currents along the grooves of the oral arms to the stomach. An Aurelia medusa clears the larger plankton from 700ml of water in less than one hour.
In Cassiopeia (a rhizostome) the tentacular fringe, aided by the vesicular appendages catches small animals and bits of flesh. These vesicles shoot out bags of nematocysts and mucus at small crustaceans. The food is entangled in the mucus and swept inward by flagellary action. After food capture, the mouths open widely and the flagellary currents carry food into the brachial canals. The term ‘suctorial mouths’ is apparently a misnomer, since there is no evidence of sucking action. In the brachial canals, ingoing currents run along the floor, and outgoing currents along the roof. The ingoing currents carry the food containing mucus strands to the gastric filaments, which grasp and pull the mucus strands into the stomach. Waste and rejected particles are carried outward in the brachial canals and emitted via the mouths. Extracellular digestion takes place in the stomach. The gastric filaments secrete an acid secretion containing proteases. The acid brings the alkaline interior fluid almost to neutrality for enzyme action. Polypeptide products of digestion are phagocytosed by the gastric filaments and also by four highly folded areas in the stomach floor (the so-called plaited membranes) which are peculiar to Cassiopeia. Intracellular digestion ensues and amoebocytes distribute food around the body. Waste is excreted into the stomach by the gastric filaments and plaited membranes, and leaves the animal via the mouths.
Ciliary currents circulate fluid in the gastrovascular system. In ephyrae these gastrovascular currents are directed outward along the roof and inward along the floor. In Aurelia medusae the currents run outward along the eight straight adradial canals and then along the ring canal and from there along the branched canals to the stomach. Waste is carried away on outward currents on the oral arms. Cyanea has no ring canal and currents flow toward the periphery along the roof and centrally along the floor of the wide gastrovascular channels, assisted by bell contractions.
Respiration is aerobic, but oxygen consumption is low because of the low organic content. Oxygen consumption per unit mass decreases as body size increases. For example, a 27.5g Aurelia aurita has been measured to consume 0.07 cm3 of oxygen per hour (0.0025 cm3 g-1 h-1), whilst an 87g individual consumed 0.17 cm3 per hour (0.0020 cm3 g-1 h-1).
In some scyphozoans the radial canals open on the subumbrellar surface near to the bell margin via excretory pores. Dissolved nitrogenous wastes and granules (?) are emitted from these pores.
Most scyphozoans are dioecious, but a few are hermaphrodite, e.g. Chrysaora. The sex cells originate and ripen in the base of the gastrodermis, which forms epithelium over the sex cells.
If septa are present, then gonads are borne on both sides of each septum such that there are eight gonads in total. These gonads are elongated or looped folded bodies that project into the gastric pockets – two gonads per pocket.
Semaeostomes and rhizostomes lack septa and the gonads are situated in the floors of the gastric pockets, above the subgenital pits and the gonads are usually four curved and folded bodies, but may hang down as bags. In some rhizostomes the four subgenital pits fuse into a single cavity, the subgenital porticus, beneath the central stomach. The four original openings are retained and the gonads are located above the porticus and often hang down into it. Sex cells are possibly discharged into the porticus (?) and the porticus may act as a brood chamber. In all other Scyphozoa the sex cells rupture into the gastrovascular cavity and escape via the mouth.
The eggs develop in seawater or in folds of the oral arms into the coeloblastula stage. The coeloblastula gives rise to a solid or hollow planula. The planula attaches and develops into a tentaculate polypoid larva or scyphistoma. The scyphistoma has a stalked, trumpet-shaped body and is fastened aborally by an adhesive disc and has four septa and four subumbrellar funnels. This stage is often followed by a free-living young medusan sage, the ephyra or ephyrula. The ephyra transforms into the adult medusa.
Asexual reproduction in scyphozoans occurs in the larval stages only. Prior to tentacle formation, Stauromedusae larvae put out 1-4 stolons that detach and become vermiform creeping larvae, which subsequently attach and develop like the original planula-derived larva.
In semaeostomes and rhizostomes the scyphistomae grow buds from the side of the stalk, in a hydra-like fasion or send out hollow stolons from the stalk or base. These stolons bud off one or more scyphistomae and then detach. If the stolons are severed before they bud, then they develop directly into scyphistomes. Fragments left behind when scyphistomae move about regenerate into scyphistomae. Some strobilae constrictions give rise to scyphistomae instead of ephyrae. In Cassiopeia the upper part of the scyphistome stalk constricts of ciliated buds that become planula-like larvae. In Chrysaora the pedal disc may secrete a chitinous cyst (pedal cyst / podocyst) that contains a ciliated larva (which has not been observed to hatch naturally?). Overfed scyphistomae cast-off their tentacles, which round up and become ciliated pseudoplanulae that develop into scyphistomae. The original ‘depressed’ scyphistome may recover or disintegrate.
Reduction phenomena can occur in all stages. When scyphistomae are starved they cease strobilation and may reduce to a clavate object. If ephyrae are starved, then they cast off their lappets and rhopalia and reduce through a gastrula-like stage to a planula-like form, which may attach. When adult medusae are starved they lose up to 80% of their weight (mostly collenchyme is lost) and gonads, rhopalia, tentacles and oral arms may dedifferentiate. In Cassiopeia the lappets become blunted, the mouth arms become stumps and the arm mouth openings fuse over.
In half bells and bells missing sectors the cut edges fuse together without replacing the missing parts. Parts of the margin, rhopalia and oral arms, etc. can regenerate. The oral mass of rhizostomes can regenerate completely. Marginal excissions in Cassiopeia regenerate quicker when rhopalia are present than if they are removed.
Regeneration studies have been carried out on the Stauromedusae: Lucernaria, Haliclystus and Thaumantoscyphus. When cut in two, the oral piece regenerates a stalk, unless it is too short. The proximal piece regenerates the oral end (less successively if cut nearer to the pedal disc). Cross-sections regenerate an oral end distally and a pedal disc or oral end proximally, depending on the level of the section. Arm base cross-sections or umbrella pieces of the wall, form tentacles and one or more manubria in Lucernaria. Longitudinal halves of Lucernaria regenerate according to the radial plane of the section. Perradial sections close together and regenerate the missing half, whilst no replacement occurs for interradial sections. Tentacle clusters, rhopalioids, etc. are replaced if removed, sometimes in excess numbers. Small oral ends or a pedal disc grow out from cuts made in the side of Lucernaria, especially if the cut involves a septum. Variations in the number of septa, lappets, gonads, rhopalia, canals, tentacles and oral arms, etc. are common and possibly arise from regeneration after injury or are congenital?
Rhythmic pulsations of the bell occur at the rate of 20-100 per minute. The resulting motion alternates between swimming and sinking, as part of the food-catching behaviour. Some scyphistome medusae weave back and forth with their tentacles spread wide.
Cassiopeia lies upside-down on the bottom of shallow lagoons, adhering by the sucker-like action of the raised circular zone on the bell (formed of tall epidermal cells). Pulsations of the bell cause water currents to flow over the mouth arms, assisting respiration and feeding.
Bell contractions are faster in warm temperatures and in younger / smaller animals, and are slower in starved medusae. Rough handling, injury or an increase in temperature causes an elevation in pulsation rate in an escape reaction.
The pulsation cycle is as follows. The radial muscles contract first causing the bell to shorten and bulge, then the coronal muscle contracts, causing the margin to contract and force water out beneath the bell; a type of jet propulsion. Contractions are synchronous over the bell.
Coordination of Locomotion
Extirpation of rhopalia abolishes or reduces the rate of contractions, temporarily or permanently. One rhopalium is sufficient to maintain more or less normal activity. The subumbrellar nerve net transmits the impulses to contract. Bell pulsations originate in the subumbrellar nerve net. Each rhopalium discharges at its own rate, the one with the fastest rate determines the rate of bell pulsation. Different rhopalia dominate at different times, since their discharge rate is variable. Medusae exhibit a righting reflex, which uses sensory input from the statocysts. In Cubomedusae the pedalia have a steering function and if they are extirpated then the medusae are no longer able to swim in a directed manner.
Many scyphozoans have symbiotic zoochlorellae or zooxanthellae living in their collenchyme, for example Cassiopeia. The photosynthesis of these organisms supplies surplus carbohydrates to the jellyfish. These symbionts are not essential and are ejected during starvation.
Some fish and crustaceans shelter under jellyfish, and many feed off them without getting stung. A spider crab, Labinia, seems to live only on the manubrium of Stomolophus.
Scyphozoa can endure wide ranges of temperature (from –0.6oC to 31oC in Aurelia, which has an optimum temperature range of 9oC to 19oC). They can also endure wide ranges of salinity (Aurelia can live in brackish water at 20% of sea water salinity) and pH (Aurelia can endure a pH range of 7.2 to 9.5, compared to the normal range of sea water, which is 8.0 to 8.2).
Some Scyphozoa luminesce.
The semaeostomes are the most familiar and typical scyphistome jellyfish and are usually the only ones seen in temperate zones. The bell is flat, saucer- or bowl-like and the margin may be scalloped into eight or more lappets. Eight to sixteen rhopalia occur in some or all of the niches between the lappets, but there are irregular numbers in some species. There are 1, 3, 5, 7 or many tentacles between successive rhopalia. The tentacles may be set into niches, or borne on lappets or on the subumbrella. Aurelia has many marginal tentacles. Some species have many tentacles arranged in U- or V-shaped bunches, e.g. Cyanea. The mouth has four angles, which are drawn out into four long, frilly, pointed or rounded lobes called oral arms. Each oral arm has an open trough and a thick gelatinous axis.
The adults have no septa, no subumbrellar funnels and no gastric pockets. There may be four subgenital pits on the subumbrellar surface beneath the gonads. A short manubrium leads into the stomach, the periphery of which may be scalloped into pouches. Numerous gastric filaments, in interradial bunches or bands, spring from the floor or periphery of the stomach. Numerous simple or branched radial canals or broad channels run to the bell margin where they branch to the rhopalia and tentacles. A ring canal is generally absent, but is present in Aurelia.
Semaestomes are found in all coastal waters, in all oceans and zones, but especially warm and temperate regions, though some are also polar, like Cyanea. Pelagia has no fixed larval stage and inhabits the open ocean. Semaeostomes are of moderate to large size, ranging from 5cm bell-diameter to over 2 m in Cyanea arctica. A variety of colourings are exhibited, there are often spots or streaks and often the gonads are more strongly coloured.
The Pelagidae bear single tentacles in niches between lappets and have simple radial canals and no ring canal. E.g. Pelagia, has 16 marginal lappets and 8 alternating tentacles and rhopalia. Chrysaora has 32 lappets, 8 rhopalia and 3 tentacles between successive rhopalia. Dactylometra inhabits warmer waters and has 48 lappets, 8 rhopalia and 5 tentacles between successive rhopalia. Sonderia has 16 rhopalia alternating with 16 tentacles.