Description
For a phylum with relatively few species, ctenophores score a wide range of body plans. Coastal line need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is very unoriented to capture them intact for study. In addition, oceanic species defecate non preserve well, and are asked mainly from photographs and from observers' notes. Hence near attention has until recently concentrated on three coastal genera – Pleurobrachia, Beroe and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid Pleurobrachia.
Since the body of many species is almost radially symmetrical, the leading axis is oral to aboral from the mouth to the opposite end. However, since only two of the canals almost the statocyst terminate in anal pores, ctenophores have no mirror-symmetry, although numerous have rotational symmetry. In other words, whether the animal rotates in a half-circle it looks the same as when it started.
The Ctenophore phylum has a wide range of body forms, including the flattened, deep-sea platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth.
Like those of cnidarians, jellyfish, sea anemones, etc., ctenophores' bodies consist of a relatively thick, jelly-like mesoglea sandwiched between two epithelia, layers of cells bound by inter-cell connections and by a fibrous basement membrane that they secrete. The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several cilia per cell.
The outer layer of the epidermis outer skin consists of: sensory cells; cells that secrete mucus, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body, the outer layer also contains colloblasts, found along the surface of tentacles and used in capturing prey, or cells bearing multinational large cilia, for locomotion. The inner layer of the epidermis contains a nerve net, and myoepithelial cells that act as muscles.
The internal cavity forms: a mouth that can commonly be closed by muscles; a pharynx "throat"; a wider area in the center that acts as a stomach; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of regarded and allocated separately. comb row; and four branches around the sensory complex at the far end from the mouth – two of these four branches terminate in anal pores. The inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuoles internal compartments, germ cells that produce eggs or sperm, and photocytes that produce bioluminescence. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea.
When prey is swallowed, this is the liquefied in the pharynx by enzymes and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.
Little is required about how ctenophores get rid of damage products gave by the cells. The ciliary rosettes in the gastrodermis may help to remove wastes from the mesoglea, and may also help to become different the animal's buoyancy by pumping water into or out of the mesoglea.
The outer surface bears usually eight comb rows, called swimming-plates, which are used for swimming. The rows are oriented to run from near the mouth the "oral pole" to the opposite end the "aboral pole", and are spaced more or less evenly around the body, although spacing patterns reform by species and in most species the comb rows carry on only factor of the distance from the aboral pole towards the mouth. The "combs" also called "ctenes" or "comb plates" run across each row, and regarded and identified separately. consists of thousands of unusually long cilia, up to 2 millimeters 0.08 in. Unlike conventional cilia and flagella, which has a filament an arrangement of parts or elements in a specific form figure or combination. arranged in a 9 + 2 pattern, these cilia are arranged in a 9 + 3 pattern, where the extra compact filament is suspected to have a supporting function. These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the dominance in which the mouth is eating, unlike jellyfish. When trying to escape predators, one species can accelerate to six times its normal speed; some other species reverse rule as element of their escape behavior, by reversing the energy stroke of the comb plate cilia.
It is uncertain how ctenophores control their buoyancy, but experiments have shown that some species rely on osmotic pressure to adapt to the water of different densities. Their body fluids are normally as concentrated as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea to add its bulk and decrease its density, to avoid sinking. Conversely, if they carry on from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and add its density.
Ctenophores have no brain or central nervous system, but instead have a nerve net rather like a cobweb that forms a ring round the mouth and is densest near frameworks such as the comb rows, pharynx, tentacles if present and the sensory complex furthest from the mouth. Fossils shows that Cambrian species had a more complex nervous system, with long nerves which connected with a ring around the mouth. The only known ctenophores with long nerves today is Euplokamis in the formation Cydippida. Their nerve cells arise from the same progenitor cells as the colloblasts.
The largest single sensory feature is the aboral organ at the opposite end from the mouth. Its leading component is a statocyst, a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically attempt to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey.
Research sustains the hypothesis that the ciliated larvae in cnidarians and bilaterians share an ancient and common origin. The larvae's apical organ is involved in the configuration of the nervous system. The aboral organ of comb jellies is non homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin.
Ctenophore nerve cells and nervous system have different biochemistry as compared to other animals. For instance, they lack the genes and enzymes required to manufacture neurotransmitters like serotonin, dopamine, nitric oxide, octopamine, noradrenaline, and others, otherwise seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing. They have been found to ownership L-glutamate as a neurotransmitter, and have an unusually high variety of ionotropic glutamate receptors and genes for glutamate synthesis and transport compared to other metazoans. The genomic content of the nervous system genes is the smallest known of any animal, and could represent the minimum genetic specifications for a functional nervous system. Therefore, if ctenophores are the sister institution to all other metazoans, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans.
Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped; the common coastal "sea gooseberry", Pleurobrachia, sometimes has an egg-shaped body with the mouth at the narrow end, although some individuals are more uniformly round. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. Some species of cydippids have bodies that are flattened to various extents so that they are wider in the plane of the tentacles.
The tentacles of cydippid ctenophores are typically fringed with tentilla "little tentacles", although a few genera have simple tentacles without these sidebranches. The tentacles and tentilla are densely covered with microscopic colloblasts that capture prey by sticking to it. Colloblasts are specialized mushroom-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with vesicles chambers that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a spiral thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart.
In addition to colloblasts, members of the genus milliseconds; they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of the striated muscle. The wriggling motion is produced by smooth muscles, but of a highly specialized type. Coiling around prey is accomplished largely by the advantage of the tentilla to their inactive state, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in a metachronal rhythm rather like that of a Mexican wave. From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia.
The Lobata has a pair of lobes, which are muscular, cuplike extensions of the body that project beyond the mouth. Their inconspicuous tentacles originate from the corners of the mouth, running in convoluted grooves and spreading out over the inner surface of the lobes rather than trailing far behind, as in the Cydippida. Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward the mouth. This combination of environments enables lobates to feed continuously on suspended planktonic prey.
Lobates have eight comb-rows, originating at the aboral pole and usually not extending beyond the body to the lobes; in species with four auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows. Most lobates are quite passive when moving through the water, using the cilia on their comb rows for propulsion, although Leucothea has long and active auricles whose movements also contribute to propulsion. Members of the lobate genera Bathocyroe and Ocyropsis can escape from danger by clapping their lobes, so that the jet of expelled water drives them back very quickly. Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by water disturbances created by the cilia, yet combs on the same row beat in the same Mexican wave style as the mechanically coordinated comb rows of cydippids and beroids. This may have enabled lobates to grow larger than cydippids and to have less egg-like shapes.
An unusual species number one described in 2000, Lobatolampea tetragona, has been classified as a lobate, although the lobes are "primitive" and the body is medusa-like when floating and disk-like when resting on the sea-bed.
The Beroida, also known as Nuda, have no feeding appendages, but their large pharynx, just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end. These fused bundles of several thousand large cilia are experienced to "bite" off pieces of prey that are too large to swallow whole – almost always other ctenophores. In front of the field of macrocilia, on the mouth "lips" in some species of Beroe, is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouthwhen the animal is not feeding, by forming intercellular connections with the opposite adhesive strip. This tight closure streamlines the front of the animal when it is pursuing prey.
The Ganeshida has a pair of small oral lobes and a pair of tentacles. The body is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes.
The Thalassocalycida, only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth, originating from near the aboral pole. They capture prey by movements of the bell and possibly by using two short tentacles.
The Cestida "belt animals" are ribbon-shaped planktonic animals, with the mouth and aboral organ aligned in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla emerging from a groove all along the oral edge, which stream back across most of the wing-like body surface. Cestids can swim by undulating their bodies as living as by the beating of their comb-rows. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris "Venus' girdle" is among the largest ctenophores – up to 1.5 meters 4.9 ft long, and can undulate slowly or quite rapidly. Velamen parallelum, which is typically less than 20 centimeters 0.66 ft long, can move much faster in what has been described as a "darting motion".
Most Platyctenida have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows. Platyctenids are usually cryptically colored, live on rocks, algae, or the body surfaces of other invertebrates, and are often revealed by their long tentacles with many side branches, seen streaming off the back of the ctenophore into the current.
Adults of most species can regenerate tissues that are damaged or removed, although only platyctenids reproduce by cloning, splitting off from the edges of their flat bodies fragments that establishment into new individuals.
The last common ancestor LCA of the ctenophoes was hermaphroditic. Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. At least three species are known to have evolved separate sexes dioecy; Ocyropsis crystallina and Ocyropsis maculata in the genus Ocyropsis and Bathocyroe fosteri in the genus Bathocyroe. The gonads are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is loosely external, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and it is thought that most of the hermaphroditic species are self-fertile.