Phylum Ctenophora
Phylum Ctenophora
Ctenophora (te-nof´o-ra) (Gr. kteis, ktenos, comb, + phora, pl. of bearing) is composed of fewer than 100 species. All are marine forms occurring in all seas but especially in warm waters. They take their name from the eight rows of comblike plates they bear for locomotion. Common names for ctenophores are “sea walnuts” and “comb jellies.” Ctenophores, along with cnidarians, represent the only two phyla having primary radial symmetry, in contrast to other metazoans, which have primary bilateral symmetry.
Ctenophores do not have nematocysts, except in one species (Haeckelia rubra, after Ernst Haeckel, nineteenthcentury German zoologist) that carries nematocysts on certain regions of its tentacles but lacks colloblasts. These nematocysts are apparently appropriated from cnidarians on which it feeds.
Like cnidarians, ctenophores have not advanced beyond the tissue grade of organization. There are no definite organ systems in the strict meaning of the term.
Except for a few creeping and sessile forms, ctenophores are freeswimming. Although they are feeble swimmers and are more common in surface waters, ctenophores are sometimes found at considerable depths. They are often at the mercy of tides and strong currents, but they avoid storms by swimming into deeper water. In calm water they may rest vertically with little movement, but when moving they use their ciliated comb plates to propel themselves mouth-end forward. Highly modified forms such as Cestum (L. cestus, girdle) use sinuous body movements as well as their comb plates in locomotion.
The fragile, transparent bodies of ctenophores are easily seen at night when they emit light (luminesce).
Classification of Phylum Cnidaria
Strong molecular and morphological evidence now indicates that members of the former phylum Myxozoa, commonly occurring fish parasites, are in fact highly derived cnidarians.* At this time, we cannot place them with confidence in the following classification; it is possible they are hydrozoans, or they should constitute a separate class.
Class Hydrozoa (hi-dro-zo´a) (Gr. hydra, water serpent, + zoon, animal). Solitary or colonial; asexual polyps and sexual medusae, although one type may be suppressed; hydranths with no mesenteries; medusae (when present) with a velum; both freshwater and marine. Examples: Hydra, Obelia, Physalia, Tubularia.
Class Scyphozoa (si-fo-zo´a) (Gr. skyphos, cup, + zoon, animal). Solitary; polyp stage reduced or absent; bellshaped medusae without velum; gelatinous mesoglea much enlarged; margin of bell or umbrella typically with eight notches that are provided with sense organs; all marine. Examples: Aurelia, Cassiopeia, Rhizostoma.
Class Cubozoa (ku´bo-zo´a) (Gr. kybos, a cube + zoon, animal). Solitary; polyp stage reduced; bell-shaped medusae square in cross section, with tentacle or group of tentacles hanging from a bladelike pedalium at each corner of the umbrella; margin of umbrella entire, without velum but with velarium; all marine. Examples: Tripedalia, Carybdea, Chironex, Chiropsalmus.
Class Anthozoa (an-tho-zo´a) (Gr. anthos, flower, + zoon, animal). All polyps; no medusae; solitary or colonial; gastrovascular cavity subdivided by at least eight mesenteries or septa bearing nematocysts; gonads endodermal; all marine.
Subclass Zoantharia (zo´an-tha´re-a) (N. L. from Gr. zoon, animal, + anthos, flower, + L. aria, like or connected with) (Hexacorallia). With simple unbranched tentacles; mesenteries in pairs; sea anemones, hard corals, and others. Examples: Metridium, Anthopleura, Tealia, Astrangia, Acropora.
Subclass Ceriantipatharia (se´re-anti-ip´a-tha´re-a) (N. L. combination of Ceriantharia and Antipatharia). With simple unbranched tentacles; mesenteries unpaired; tube anemones and black or thorny corals. Examples: Cerianthus, Antipathes, Stichopathes.
Subclass Alcyonaria (al´ce-o-na´re-a) (Gr. alkonion, kind of sponge resembling nest of kingfisher [alkyon, kingfisher], + L. aria, like or connected with) (Octocorallia). With eight pinnate tentacles; eight complete, unpaired mesenteries; soft and horny corals. Examples: Tubipora, Alcyonium, Gorgonia, Plexaura, Renilla.
Class Tentaculata
Representative Type: Pleurobrachia
Pleurobrachia (Gr. pleuron, side, + L. brachia, arms) is a representative of this group of ctenophores. Its transparent body is about 1.5 to 2 cm in diameter (Figure 13-35A). The oral pole bears the mouth opening, and the aboral pole has a sensory organ, the statocyst.
Comb Plates: On the surface are eight equally spaced bands called comb rows, which extend as meridians from the aboral pole and end before reaching the oral pole (Figure 13-36). Each band consists of transverse plates of long fused cilia called comb plates (Figure 13-36A). Ctenophores are propelled by beating of cilia on the comb plates. The beat in each row starts at the aboral end and proceeds successively along the combs to the oral end. All eight rows normally beat in unison. The animal is thus driven forward with the mouth in advance. The animal can swim backward by reversing the direction of the wave.
Tentacles: The two tentacles are long, solid and very extensible, and they can be retracted into a pair of tentacle sheaths. When completely extended, they may measure 15 cm in length. The surface of the tentacles bears colloblasts, or glue cells (Figure 13-36C), which secrete a sticky substance that is used for catching and holding small animals.
Body Wall: The cellular layers of ctenophores are generally similar to those of cnidarians. Between the epidermis and gastrodermis is a gelatinous collenchyme that fills most of the interior of the body and contains muscle fibers and ameboid cells. Although they are derived from ectodermal cells, muscle cells are distinct and are not contractile portions of epitheliomuscular cells (in contrast to Cnidaria).
Digestive System and Feeding The gastrovascular system: consists of a mouth, a pharynx, a stomach, and a system of gastrovascular canals that branch through the jelly to extend to the comb plates, tentacular sheaths, and elsewhere (Figure 13-36). There are two blind canals that terminate near the mouth, and an aboral canal that passes near the statocyst and then divides into two small anal canals through which undigested material is expelled.
Ctenophores prey on small planktonic organisms such as copepods. Glue cells on the tentacles stick to small prey and enable the tentacles to carry the prey to the ctenophore’s mouth. Digestion is both extracellular and intracellular.
Characteristics of Phylum Ctenophora
Comparison with Cnidaria
Ctenophores resemble the cnidarians in the following ways:
Respiration and Excretion: Respiration and excretion occur through the body surface.
Nervous and Sensory Systems: Ctenophores have a nervous system similar to that of cnidarians. It features a subepidermal plexus, which is concentrated under each comb plate, but no central control as is found in more complex animals.
The sense organ at the aboral pole is a statocyst. Tufts of cilia support a calcareous statolith, with the whole being enclosed in a bell-like container. Alterations in the position of the animal change the pressure of the statolith on the tufts of cilia. The sense organ is also concerned in coordinating the beating of the comb rows but does not trigger their beat.
The epidermis of ctenophores bears abundant sensory cells, so the animals are sensitive to chemical and other forms of stimuli. When a ctenophore comes in contact with an unfavorable stimulus, it often reverses the beat of its comb plates and moves backward. Comb plates are very sensitive to touch, which often causes them to be withdrawn into the jelly.
Reproduction and Development: Pleurobrachia, like other ctenophores, is monoecious. Gonads are located on the lining of the gastrovascular canals under the comb plates. Fertilized eggs are discharged through the epidermis into the water.
Cleavage in ctenophores is determinate (mosaic), since the various parts of the animal that will be formed by each blastomere are determined early in embryogenesis. If one of the blastomeres is removed in the early stages, the resulting embryo will be deficient. This type of development differs from that of cnidarians, which is regulative (indeterminate). The freeswimming cydippid larva is superficially similar to the adult ctenophore and develops directly into an adult.
Some biologists have regarded the ctenophores and some more complex cnidarians (for example, some anthozoans) as triploblastic because the highly cellular nature of the mesoglea would constitute a mesoderm. However, others define mesoderm strictly as a layer derived from endoderm; thus both cnidarians and ctenophores would be diploblastic.
Other Ctenophores
Ctenophores are fragile and beautiful creatures. Their transparent bodies glisten like fine glass, brilliantly iridescent during the day and luminescent at night.
One of the most striking ctenophores is Beroe (L. a nymph), which may be more than 100 mm in length and 50 mm in breadth (Figure 13-37A). It is conical or thimble shaped and is flattened in the tentacular plane. The tentacular plane in Beroe is defined as where the tentacles would have been, because it has a large mouth but no tentacles. The animal is pink or rusty brown. Its body wall is covered with an extensive network of canals formed by union of the paragastric and meridional canals. Venus’ girdle (Cestum, Figure 13-37B) is highly compressed in the tentacular plane. Bandlike, it may be more than 1 m long and presents a graceful appearance as it swims in the oral direction. The highly modified Ctenoplana (Gr. ktenos, comb, + L. planus, flat) and Coeloplana (Gr. koilos, hollow, + L. planus, flat) (Figure 13-37C) are rare but are interesting because they have disc-shaped bodies flattened in the oral-aboral axis and are adapted for creeping rather than swimming. A common ctenophore along the Atlantic and Gulf coasts is Mnemiopsis (Gr. mneme, memory, + opsis, appearance) (Figure 13-35B), which has a laterally compressed body with two large oral lobes and unsheathed tentacles.
Nearly all ctenophores give off flashes of luminescence at night, especially such forms as Mnemiopsis (Figure 13-35B). The vivid flashes of light seen at night in southern seas are often caused by members of this phylum.
Classification of Phylum Ctenophora
Class Tentaculata (ten-tak´yu-la´ta) (L. tentaculum, feeler, + ata, group suffix). With tentacles; tentacles may or may not have sheaths into which they retract; some types flattened in oral-aboral axis for creeping; others compressed in tentacular plane to a bandlike form; in some the comb plates may be confined to the larval form. Examples: Pleurobrachia, Cestum.
Class Nuda (nu-da) (L. nudus, naked). Without tentacles, but flattened in tentacular plane; wide mouth and pharynx; gastrovascular canals much branched. Example: Beroe.
Ctenophora (te-nof´o-ra) (Gr. kteis, ktenos, comb, + phora, pl. of bearing) is composed of fewer than 100 species. All are marine forms occurring in all seas but especially in warm waters. They take their name from the eight rows of comblike plates they bear for locomotion. Common names for ctenophores are “sea walnuts” and “comb jellies.” Ctenophores, along with cnidarians, represent the only two phyla having primary radial symmetry, in contrast to other metazoans, which have primary bilateral symmetry.
Ctenophores do not have nematocysts, except in one species (Haeckelia rubra, after Ernst Haeckel, nineteenthcentury German zoologist) that carries nematocysts on certain regions of its tentacles but lacks colloblasts. These nematocysts are apparently appropriated from cnidarians on which it feeds.
Like cnidarians, ctenophores have not advanced beyond the tissue grade of organization. There are no definite organ systems in the strict meaning of the term.
Except for a few creeping and sessile forms, ctenophores are freeswimming. Although they are feeble swimmers and are more common in surface waters, ctenophores are sometimes found at considerable depths. They are often at the mercy of tides and strong currents, but they avoid storms by swimming into deeper water. In calm water they may rest vertically with little movement, but when moving they use their ciliated comb plates to propel themselves mouth-end forward. Highly modified forms such as Cestum (L. cestus, girdle) use sinuous body movements as well as their comb plates in locomotion.
The fragile, transparent bodies of ctenophores are easily seen at night when they emit light (luminesce).
Classification of Phylum Cnidaria
Strong molecular and morphological evidence now indicates that members of the former phylum Myxozoa, commonly occurring fish parasites, are in fact highly derived cnidarians.* At this time, we cannot place them with confidence in the following classification; it is possible they are hydrozoans, or they should constitute a separate class.
Class Hydrozoa (hi-dro-zo´a) (Gr. hydra, water serpent, + zoon, animal). Solitary or colonial; asexual polyps and sexual medusae, although one type may be suppressed; hydranths with no mesenteries; medusae (when present) with a velum; both freshwater and marine. Examples: Hydra, Obelia, Physalia, Tubularia.
Class Scyphozoa (si-fo-zo´a) (Gr. skyphos, cup, + zoon, animal). Solitary; polyp stage reduced or absent; bellshaped medusae without velum; gelatinous mesoglea much enlarged; margin of bell or umbrella typically with eight notches that are provided with sense organs; all marine. Examples: Aurelia, Cassiopeia, Rhizostoma.
Class Cubozoa (ku´bo-zo´a) (Gr. kybos, a cube + zoon, animal). Solitary; polyp stage reduced; bell-shaped medusae square in cross section, with tentacle or group of tentacles hanging from a bladelike pedalium at each corner of the umbrella; margin of umbrella entire, without velum but with velarium; all marine. Examples: Tripedalia, Carybdea, Chironex, Chiropsalmus.
Class Anthozoa (an-tho-zo´a) (Gr. anthos, flower, + zoon, animal). All polyps; no medusae; solitary or colonial; gastrovascular cavity subdivided by at least eight mesenteries or septa bearing nematocysts; gonads endodermal; all marine.
Subclass Zoantharia (zo´an-tha´re-a) (N. L. from Gr. zoon, animal, + anthos, flower, + L. aria, like or connected with) (Hexacorallia). With simple unbranched tentacles; mesenteries in pairs; sea anemones, hard corals, and others. Examples: Metridium, Anthopleura, Tealia, Astrangia, Acropora.
Subclass Ceriantipatharia (se´re-anti-ip´a-tha´re-a) (N. L. combination of Ceriantharia and Antipatharia). With simple unbranched tentacles; mesenteries unpaired; tube anemones and black or thorny corals. Examples: Cerianthus, Antipathes, Stichopathes.
Subclass Alcyonaria (al´ce-o-na´re-a) (Gr. alkonion, kind of sponge resembling nest of kingfisher [alkyon, kingfisher], + L. aria, like or connected with) (Octocorallia). With eight pinnate tentacles; eight complete, unpaired mesenteries; soft and horny corals. Examples: Tubipora, Alcyonium, Gorgonia, Plexaura, Renilla.
Class Tentaculata
Representative Type: Pleurobrachia
Pleurobrachia (Gr. pleuron, side, + L. brachia, arms) is a representative of this group of ctenophores. Its transparent body is about 1.5 to 2 cm in diameter (Figure 13-35A). The oral pole bears the mouth opening, and the aboral pole has a sensory organ, the statocyst.
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| Figure 13-35 A, Comb jelly Pleurobrachia sp. (order Cydippida, class Tentaculata). Its fragile beauty is especially evident at night when it luminesces from its comb rows. B, Mnemiopsis sp. (order Lobata, class Tentaculata). |
Comb Plates: On the surface are eight equally spaced bands called comb rows, which extend as meridians from the aboral pole and end before reaching the oral pole (Figure 13-36). Each band consists of transverse plates of long fused cilia called comb plates (Figure 13-36A). Ctenophores are propelled by beating of cilia on the comb plates. The beat in each row starts at the aboral end and proceeds successively along the combs to the oral end. All eight rows normally beat in unison. The animal is thus driven forward with the mouth in advance. The animal can swim backward by reversing the direction of the wave.
Tentacles: The two tentacles are long, solid and very extensible, and they can be retracted into a pair of tentacle sheaths. When completely extended, they may measure 15 cm in length. The surface of the tentacles bears colloblasts, or glue cells (Figure 13-36C), which secrete a sticky substance that is used for catching and holding small animals.
 |
| Figure 13-36 Comb jelly Pleurobrachia, a ctenophore. A, External view. B, Hemisection. C, Colloblast, an adhesive cell characteristic of ctenophores. D, Portion of comb rows showing comb plates, each composed of transverse rows of long fused cilia. |
Body Wall: The cellular layers of ctenophores are generally similar to those of cnidarians. Between the epidermis and gastrodermis is a gelatinous collenchyme that fills most of the interior of the body and contains muscle fibers and ameboid cells. Although they are derived from ectodermal cells, muscle cells are distinct and are not contractile portions of epitheliomuscular cells (in contrast to Cnidaria).
Digestive System and Feeding The gastrovascular system: consists of a mouth, a pharynx, a stomach, and a system of gastrovascular canals that branch through the jelly to extend to the comb plates, tentacular sheaths, and elsewhere (Figure 13-36). There are two blind canals that terminate near the mouth, and an aboral canal that passes near the statocyst and then divides into two small anal canals through which undigested material is expelled.
Ctenophores prey on small planktonic organisms such as copepods. Glue cells on the tentacles stick to small prey and enable the tentacles to carry the prey to the ctenophore’s mouth. Digestion is both extracellular and intracellular.
Characteristics of Phylum Ctenophora
- Symmetry biradial; arrangement of internal canals and position of the paired tentacles change the radial symmetry into a combination of the two (radial + bilateral)
- Usually ellipsoidal or spherical in shape, with radially arranged rows of comb plates for swimming
- Ectoderm, endoderm, and a mesoglea (ectomesoderm) with scattered cells and muscle fibers; may be considered triploblastic
- Nematocysts absent but adhesive cells (colloblasts) present
- Digestive system consisting of mouth, pharynx, stomach, a series of canals, and anal pores
- Nervous system consisting of a subepidermal plexus concentrated around the mouth and beneath the comb plate rows; an aboral sense organ (statocyst)
- No polymorphism or dimorphism
- Reproduction monoecious; gonads (endodermal origin) on the walls of the digestive canals, which are under the rows of comb plates; mosaic cleavage; cydippid larva
- Luminescence common
Comparison with Cnidaria
Ctenophores resemble the cnidarians in the following ways:
- Form of radial symmetry
- Aboral-oral axis around which the parts are arranged
- Well-developed gelatinous ectomesoderm (collenchyme)
- No coelomic cavity
- Diffuse nerve plexus
- Lack of organ systems
- They do not form nematocysts
- Development of distinct muscle cells from mesenchyme
- Presence of comb plates and colloblasts
- Mosaic, or determinate type of development
- Presence of pharynx generally
- No polymorphism or dimorphism
- Never colonial
- Presence of anal openings
Respiration and Excretion: Respiration and excretion occur through the body surface.
Nervous and Sensory Systems: Ctenophores have a nervous system similar to that of cnidarians. It features a subepidermal plexus, which is concentrated under each comb plate, but no central control as is found in more complex animals.
The sense organ at the aboral pole is a statocyst. Tufts of cilia support a calcareous statolith, with the whole being enclosed in a bell-like container. Alterations in the position of the animal change the pressure of the statolith on the tufts of cilia. The sense organ is also concerned in coordinating the beating of the comb rows but does not trigger their beat.
The epidermis of ctenophores bears abundant sensory cells, so the animals are sensitive to chemical and other forms of stimuli. When a ctenophore comes in contact with an unfavorable stimulus, it often reverses the beat of its comb plates and moves backward. Comb plates are very sensitive to touch, which often causes them to be withdrawn into the jelly.
Reproduction and Development: Pleurobrachia, like other ctenophores, is monoecious. Gonads are located on the lining of the gastrovascular canals under the comb plates. Fertilized eggs are discharged through the epidermis into the water.
Cleavage in ctenophores is determinate (mosaic), since the various parts of the animal that will be formed by each blastomere are determined early in embryogenesis. If one of the blastomeres is removed in the early stages, the resulting embryo will be deficient. This type of development differs from that of cnidarians, which is regulative (indeterminate). The freeswimming cydippid larva is superficially similar to the adult ctenophore and develops directly into an adult.
Some biologists have regarded the ctenophores and some more complex cnidarians (for example, some anthozoans) as triploblastic because the highly cellular nature of the mesoglea would constitute a mesoderm. However, others define mesoderm strictly as a layer derived from endoderm; thus both cnidarians and ctenophores would be diploblastic.
Other Ctenophores
Ctenophores are fragile and beautiful creatures. Their transparent bodies glisten like fine glass, brilliantly iridescent during the day and luminescent at night.
One of the most striking ctenophores is Beroe (L. a nymph), which may be more than 100 mm in length and 50 mm in breadth (Figure 13-37A). It is conical or thimble shaped and is flattened in the tentacular plane. The tentacular plane in Beroe is defined as where the tentacles would have been, because it has a large mouth but no tentacles. The animal is pink or rusty brown. Its body wall is covered with an extensive network of canals formed by union of the paragastric and meridional canals. Venus’ girdle (Cestum, Figure 13-37B) is highly compressed in the tentacular plane. Bandlike, it may be more than 1 m long and presents a graceful appearance as it swims in the oral direction. The highly modified Ctenoplana (Gr. ktenos, comb, + L. planus, flat) and Coeloplana (Gr. koilos, hollow, + L. planus, flat) (Figure 13-37C) are rare but are interesting because they have disc-shaped bodies flattened in the oral-aboral axis and are adapted for creeping rather than swimming. A common ctenophore along the Atlantic and Gulf coasts is Mnemiopsis (Gr. mneme, memory, + opsis, appearance) (Figure 13-35B), which has a laterally compressed body with two large oral lobes and unsheathed tentacles.
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| Figure 13-37 Diversity among the phylum Ctenophora. A, Beroe sp. (order Beroida, class Nuda). B, Cestum sp. (order Cestida, class Tentaculata). C, Coeloplana sp. (order Platyctenea, class Tentaculata). |
Nearly all ctenophores give off flashes of luminescence at night, especially such forms as Mnemiopsis (Figure 13-35B). The vivid flashes of light seen at night in southern seas are often caused by members of this phylum.
Classification of Phylum Ctenophora
Class Tentaculata (ten-tak´yu-la´ta) (L. tentaculum, feeler, + ata, group suffix). With tentacles; tentacles may or may not have sheaths into which they retract; some types flattened in oral-aboral axis for creeping; others compressed in tentacular plane to a bandlike form; in some the comb plates may be confined to the larval form. Examples: Pleurobrachia, Cestum.
Class Nuda (nu-da) (L. nudus, naked). Without tentacles, but flattened in tentacular plane; wide mouth and pharynx; gastrovascular canals much branched. Example: Beroe.
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