The colours have been added for clarity in distinguishing the parts. In hydroid colonies, the tentacled feeding zooids or hydranths are usually white, pink or violet white in Obelia and in Obelia the chitinous outer covering or perisarc often gives the colonies a yellowish colour. Obelia is marine. Branching stolons creep along the substrate, forming the hydrorhiza , and upright stalks hydrocauli, sing, hydrocaulus shoot-up from the hydrorhiza at intervals, forming branching structures - two feeding polyps can be seen on the left, a developing polyp as a bud in the centre, and a reproductive polyp on the right.
The reproductive polyps gonangia or blastostyles generally grow from axils or angles in the colony, typically in the angle between a hydranth and its parent stalk. Colony Form The colony form depends upon species, and deviations from the usual patterns are apparent, however, the typical pattern is sympodial growth for the vertical shoots with each branch terminating in a hydranth and giving rise to another branch.
The hydrorhiza is a prostrate mass of branching tubes stolons from which vertical shoots arise at intervals, each forming a vertical stem or hydrocaulus, reaching cm in height. Gonangia develop in the axils of branches. Shown in blue are the developing medusae buds on the gonangia. Each hydranth sits on a shelf of perisarc.
The individuals making up the colony are called zooids or polyps. Like hydra, the body wall consists of an outer cell layer or ectodermis and an inner cell layer or endodermis or gastrodermis with mesogloea separating the two. The endodermis encloses a central fluid-filled cavity, the gastrodermal cavity or enteron. The tubes that make up the hydrorhiza and hydrocauli contain an inner cylinder of living tissue, the coenosarc shown as orange in the coloured picture surrounded by a hollow outer cylinder or perisarc , which is a chitinous sheath that acts as an exoskeleton supporting the colony.
At branches and beneath the polyps, the perisarc forms a series of rings or annuli which act as skeletal joints and give the colony more flexibility.
The coenosarc consists of an outer layer of ectodermal cells and an inner layer of endodermal cells enclosing the enteron, and mesogloea between the two cell layers. The enteron forms a continuous fluid-filled cavity throughout the whole colony, connected the polyps. In the hydranths the enteron is enlarged to form a stomach cavity. The coenosarc is separated by a fluid-filled space from the perisarc which is porous to sea-water and the two only make contact at intervals.
Nutrition The tentacled feeding polyps are called hydranths and the perisarc enclosing them forms a supporting cup called a hydrotheca. Each hydranth terminates in a cone hypostome bearing the mouth and surrounded by a ring of about 24 tentacles. Each hydranth is a feeding polyp that resembles a hydra. However, the tentacles are solid they are hollow in hydra and each is filled with a cord of endodermal cells with secreted matrix material separating adjacent cells.
The tentacles and the hypostome are the only parts of the colony that generally bear nematocysts stinging structures located in cells called nematoblasts, nematocytes, cnidoblasts or cnidocytes. There is only one nematocyst type in Obelia - a barbed penetrant, which fires a barbed harpoon-like thread. The nematoblasts are derived from interstitial cells.
The hypostome has the highest density of sensory cells. The tentacles catch any prey that triggers the nematocysts and which is not large enough to escape. The tentacles pass the ensnared prey to the mouth and hence into the enteron where the first-phase of digestion, which is extracellular, takes place.
The enteron of the hydranth connects to the enteron of the coenosarc and the endodermis lining the enteron contains flagellated cells. The beating of the flagella generate currents that carry the fluid containing the partially digested food around the colony. Endodermal cells throughout the colony phagocytose the food particles and complete digestion intracellularly. Soluble products of digestion are then passed to the ectodermal cells. Thus, even the gonangia, which cannot feed themselves, derive nourishment via the hydranths and the coenosarc transport system.
Reproduction Asexual reproduction has been shown to occur in some species. Shoots of the coenosarc that begin development as gonangia, instead enlarge and detach and slip out from the perisarc and float away, attach to a substrate and develop into a new colony. Sexual reproduction occurs through the medusae. Anemone fish, or clownfish, are able to live in the anemone since they are immune to the toxins contained within the nematocysts.
Another type of anthozoan that forms an important mutualistic relationship is reef building coral. These hermatypic corals rely on a symbiotic relationship with zooxanthellae.
The coral gains photosynthetic capability, while the zooxanthellae benefit by using nitrogenous waste and carbon dioxide produced by the cnidarian host. Anthozoans remain polypoid throughout their lives. They can reproduce asexually by budding or fragmentation, or sexually by producing gametes.
Both gametes are produced by the polyp, which can fuse to give rise to a free-swimming planula larva. The larva settles on a suitable substratum and develops into a sessile polyp. Scyphozoans are free-swimming, polymorphic, dioecious, and carnivorous cnidarians with a prominent medusa morphology.
Class Scyphozoa, an exclusively marine class of animals with about known species, includes all the jellies. The defining characteristic of this class is that the medusa is the prominent stage in the life cycle, although there is a polyp stage present.
Members of this species range from 2 to 40 cm in length, but the largest scyphozoan species, Cyanea capillata , can reach a size of 2 m across. Scyphozoans display a characteristic bell-like morphology.
Scyphozoans : For jellyfish a , and all other scyphozoans, the medusa b is the most prominent of the two life stages. In the jellyfish, a mouth opening, surrounded by tentacles bearing nematocysts, is present on the underside of the animal. Scyphozoans live most of their life cycle as free-swimming, solitary carnivores. The mouth leads to the gastrovascular cavity, which may be sectioned into four interconnected sacs, called diverticuli. In some species, the digestive system may be further branched into radial canals.
Like the septa in anthozoans, the branched gastrovascular cells serves to increase the surface area for nutrient absorption and diffusion; thus, more cells are in direct contact with the nutrients in the gastrovascular cavity. In scyphozoans, nerve cells are scattered over the entire body. Neurons may even be present in clusters called rhopalia. These animals possess a ring of muscles lining the dome of the body, which provides the contractile force required to swim through water.
Scyphozoans are dioecious animals, having separate sexes. The gonads are formed from the gastrodermis with gametes expelled through the mouth. Planula larvae are formed by external fertilization; they settle on a substratum in a polypoid form known as scyphistoma. These forms may produce additional polyps by budding or may transform into the medusoid form. The life cycle of these animals can be described as polymorphic because they exhibit both a medusal and polypoid body plan at some point.
Lifecycle of a jellyfish : The lifecycle of a jellyfish includes two stages: the medusa stage and the polyp stage. The polyp reproduces asexually by budding,while the medusa reproduces sexually. Cubozoans live as box-shaped medusae while Hydrozoans are true polymorphs and can be found as colonial or solitary organisms. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans.
A prominent difference between the two classes is the arrangement of tentacles. This is the most venomous group of all the cnidarians. Cubozoans : The a tiny cubazoan jelly Malo kingi is thimble shaped and, like all cubozoan jellies, b has four muscular pedalia to which the tentacles attach.
Two people in Australia, where Irukandji jellies are most-commonly found, are believed to have died from Irukandji stings. The cubozoans contain muscular pads called pedalia at the corners of the square bell canopy, with one or more tentacles attached to each pedalium.
These animals are further classified into orders based on the presence of single or multiple tentacles per pedalium. In some cases, the digestive system may extend into the pedalia. Nematocysts may be arranged in a spiral configuration along the tentacles; this arrangement helps to effectively subdue and capture prey.
Cubozoans exist in a polypoid form that develops from a planula larva. These polyps show limited mobility along the substratum.
As with scyphozoans, they may bud to form more polyps to colonize a habitat. Polyp forms then transform into the medusoid forms. Hydrozoa includes nearly 3, species; most are marine, although some freshwater species are known. Animals in this class are polymorphs: most exhibit both polypoid and medusoid forms in their lifecycle, although this is variable. The polyp form in these animals often shows a cylindrical morphology with a central gastrovascular cavity lined by the gastrodermis. The gastrodermis and epidermis have a simple layer of mesoglea sandwiched between them.
A mouth opening, surrounded by tentacles, is present at the oral end of the animal. Obelia has only one kind of nematocysts called basitrichous isorhizas in which the capsule is oval, butt is absent, the thread is open at the tip and has spines on its base. It has long, granular epitheliomuscular cells, their muscle processes point outwards and are circular. Endoderm cells have flagella which produce a current in the enteron. They can also form pseudopodia for engulfing food.
The endoderm of tentacles has cubical, vacuolated cells with thick walls. In the endoderm layer are nerve cells and club-shaped gland cells which produce digestive enzymes. Mesogloea is a thin jelly-like substance with no structure or cells.
On each side of the mesogloea is a nerve net composed of nerve cells and their fibres, the two nerve nets are inter-connected. Polyp is the nutritive zooid of the colony. It is carnivorous and feeds upon aquatic crustaceans, nematodes and other worms. Tentacles help in catching and conveying the prey to the mouth. Digestive juice is secreted in the gland cells of gastro dermis and the process of digestion is extracellular as well as intracellular. The gonangium Gr.
It is covered by a transparent gonotheca and contains an axis or blastostyle on which lateral buds form that develop into medusae or gonophores. The blastostyle has no mouth and no tentacles, but ends distally into a flattened disc. The gonotheca opens at its distal end by a gonopore, through which the medusae escape. Gonotheca, blastostyle and the gonophores together form a gonangium.
This is a special mode of asexual reproduction. The medusa is a modified zooid produced as a hollow bud from the coenosarc of the blastostyle in spring and summer. Medusa swims freely on the surface water. It is saucer-shaped, it is attached by the middle of the convex surface to the blastostyle, when fully formed it breaks free and emerges from the mouth of the gonotheca.
The medusa is circular and tiny umbrella-like in shape. The convex outer surface is known as the ex-umbrella and the concave inner surface is the sub-umbrella. From the centre of the sub-umbrella arises a short projecting manubrium L. The mouth leads into an enteric cavity or gastric cavity in the manubrium. From the enteric cavity, arise four radial canals which are delicate ciliated tubes, they run to the margin of the bell to join a ciliated circular canal running near the margin.
The enteric cavity and the canals represent the enteron which distributes food. Projecting from the middle of the radial canals are four gonads, since sexes are separate they are either four testes or four ovaries,they are patches of modified sub-umbrellar ectoderm. The gonads mature after the medusae escape from the gonotheca. The edge of the bell is produced inwards as a thin fold called velum. Velum is characteristic of hydrozoan medusae but it is insignificant in Obelia.
The medusae with a velum are called craspedote, and those with no velum are acraspedote Scyphozoa. From the edge of the bell numerous small solid tentacles hang downwards. The tentacles have swollen bases due to the accumulation of interstitial cells which are practically absent from other regions.
The basal swellings of tentacles are called vesicles or bulbs, nematocysts are formed continuously in the bulbs from where they migrate to the tentacles. Digestive enzymes are secreted from the endoderm of bulbs. Near the bulbs the ectoderm has pigment granules and nerve cells, they are often called ocelli and it is claimed that the ocelli are sensory to light, but more probably there are no ocelli, the pigment granules are accumulated excretory matter.
Above the bulb of every tentacle is a tiny fluid-filled vesicle. Nematocysts are confined to the manubrium and tentacles, there may be some on the bell margin.
There are eight marginal sense organs called statocysts or lithocysts lying at regular intervals, being attached on the sub-umbrella side to the bulbs of eight tentacles, they are developed in response to a locomotory habit. A statocyst is a tiny, circular closed vesicle lined with ectoderm and filled with a fluid containing calcareous granules called otoliths which lie in a special cell called lithocyte.
The lining has some sensory cells with thin sensory processes on which the otoliths produce a stimulus which is transmitted by nerves to muscles; the muscles coordinate the snake-like swimming movements of the medusa, and should the medusa become tilted, the muscles contract to right the position of medusa bell, thus, statocysts are balancing organs. The ectoderm covers the bell on all sides, its epitheliomuscular cells are produced into muscle processes which run longitudinally in the manubrium and tentacles.
In the sub-umbrella, the muscle processes of the ectoderm are so large in proportion to the epithelial part that they almost form muscles only. The muscle processes form a striated circular muscle and some radial muscles in the sub-umbrella, they bring about locomotory movements.
The ectoderm of the ex-umbrella is devoid of musculature. The endoderm lines the enteric cavity and the radial and circular canals.
The endoderm cells have no muscle processes, they are ciliated epithelial cells, they are digestive. Between the two ectoderm layers of the bell is a thin sheet of endoderm lamella except where the enteron lies.
The endoderm lamella is formed by the fusion of upper and lower layers of endoderm, the fusion having occurred at all places except in the region of the enteron. Between the ectoderm and endoderm is thick mesogloea forming the bulk of the bell of the medusa, manubrium and tentacles. The velum has a double layer of ectoderm and the thick mesogloea in the middle, it has no endoderm.
The presence of the four radial canals distinguishes the four principal radii or per-radii. Halfway between any two per-radii a radius of the second order or inter-radius may be taken. Halfway between any per-radius and inter-radius on either side a radius of third order, or ad-radius, and halfway between any ad-radius and the adjacent per- or inter-radius, a radius of fourth order or sub-radius. Thus, there are four per-radii, four inter-radii, eight ad-radii and sixteen sub-radii.
In Obelia, the radial canals, the angles of the mouth and four of the tentacles are the per-radial, four more tentacles are inter- radial, and the remaining eight tentacles, bearing the lithocysts are ad-radial. Sub-radii are of no importance in this particular form. The blastostyle produces medusae by budding in large numbers. The cavity of the blastostyle pushes the coenosarc out to form a small protuberance or bud. The bud grows larger and its coenosarc becomes like a vesicle which is attached to the blastostyle by a narrow stalk.
The cavity of the vesicle is continuous with the enteron of blastostyle. The distal ectoderm of the vesicle separates into two layers, then the inner layer of ectoderm splits to acquire a cavity called a bell rudiment. There are now two layers of ectoderm outside the bell rudiment and one layer on the inner side. The cavity of the bell rudiment assumes the shape of the sub-umbrella, and a manubrium is formed in the centre.
The two layers of ectoderm which enclose the bell rudiment from outside now break leaving a marginal and circular shelf called velum. The manubrium acquires a mouth, marginal tentacles are formed, the stalk breaks and its aperture closes up, thus, a medusa is formed which is set free, it escapes from the gonotheca, later its gonads mature.
Medusae are carnivorous and the processes of their nutrition are essentially the same as in the polyp. The food consists of living animals or bodies of animals. Digestion is both extracellular and intracellular. Extracellular digestion occurs in the main part of the gastro vascular cavity and is purely proteolytic.
Hyman has shown that although food particles are distributed throughout the gastro vascular cavity, most intracellular digestion takes place in the manubrium, in the stomach and in tentacular bulbs. The digested food is distributed to the entire medusa through the system of radial and circular canals.
The muscular system of medusa is somewhat more specialised than in the polyp. The gastro dermal cells lack contractile extensions, and the muscular system is, thus, restricted to the epidermal layer. Furthermore, the muscular system is best developed around the bell margin and sub umbrella surface where the fibres form a radial and circular system. Some of the epitheliomuscular cells of the velum have their contractile extensions oriented to form a powerful circular band of fibres which are striated.
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