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In this article we will discuss about Ctenophora: 1. Taxonomic History of Ctenophora 2. Taxonomic Retrospect of Ctenophora 3. Definition 4. Features 5. Course of Development 6. Scheme of Classification 7. Affinities 8. Ancestry.
Contents:
- Taxonomic History of Ctenophora
- Taxonomic Retrospect of Ctenophora
- Definition of Ctenophora
- Features of Ctenophora
- Course of Development of Ctenophora
- Scheme of Classification of Ctenophora
- Affinities of Ctenophora
- Ancestry of Ctenophora
1. Taxonomic History of Ctenophora:
In the field of taxonomic zoology, Ctenophora, for a long time, occupies a very important place due to its peculiar organization.
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(i) Martens, a doctor, first discovered ctenophores in 1671. Linnaeus placed the animals under the group zoophyta.
(ii) Eschscholtz (1829) first created the order Ctenophora under Coelenterata.
(iii) Leuckart (1847-1848) included the sponges amongst the ctenophores.
(iv) Hatschek (1889) gave a separate status to ctenophores as a group (Phylum).
2. Taxonomic Retrospect of Ctenophora:
(i) The ctenophores are recognised since 1671. Linnaeus placed two ctenophores under the group zoophyta.
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(ii) Eschscholtz (1829) was the first zoologist to recognise the group Ctenophora. Since then the ctenophores were considered as intermediate between zoophytes and echinoderms.
(iii) Leuckart (1847-1848) first separated Coelenterata from the echinoderms but the sponges and ctenophores were included under Coelenterata.
(iv) Vasmaer (1877) separated the sponges from the coelenterates and since then it was the convention to include the ctenophores under the so-called phylum Coelenterata.
(v) Hatschek (1889) removed the ctenophores as a separate group.
Many zoologists, such as Hyman (1940), Marshall and Williams (1972), Barnes (1981), Ruppert and Barnes (1994) and Pechnik (2000) recognised two classes:
(i) Tentaculata and
(ii) Nuda under Ctenophora.
But recently the trend has reversed as the doubt of monophyletic lineage of two classes.
3. Definition of Ctenophora:
Soft, transparent, diploblastic, marine metazoans without nematocysts, but with adhesive colloblasts and eight meridional ciliated combplates, called Ctenophora.
4. Features of Ctenophora:
Characteristic Features of the Phylum:
1. Ctenophores are exclusively marine and most are planktonic.
2. Body is soft transparent and biradially symmetrical with tissue-grade organization. Body axis is oral and aboral.
3. Diploblastic metazoans.
4. The body wall consists of an outer epidermis and an inner gastro-dermis. Between the two layers there is a thick, gelatinous middle mesogloea layer containing mesenchymal muscle cells.
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5. Nematocysts or cnidae are absent except in Euchlora rubra where nematocysts are present in tentacles. These nematocysts are not produced by the ctenophore and are obtained probably through the ingestion of cnidarian medusae.
6. Eight meridional rows of comb-like ciliary plates or costae are present that propel the animal.
7. There are peculiar adhesive structures called colloblasts or lasso cells present in the tentacles, which help to kill the prey except Euchlora rubra.
8. A specialised aboral sense organ is present, and acts as equilibrium.
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9. Digestive system represents the gastro-vascular canals. Extracellular and intracellar types of digestion. Intracellular digestion takes place in gastro-vascular canals.
10. Nervous system represents by a diffused sub-epidermal nerve net.
11. There are no respiratory, circulatory or excretory systems.
12. Hermaphroditic (mostly).
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13. Cleavage determinate and mosaic type of development.
14. Gastrulation by epiboly or invagination.
15. A distinctive cydippid larva occurs in the life cycle.
16. Nearly all ctenophores are bioluminescent.
17. All are predators and feed on other small planktonic animals.
Diagnostic Features of Ctenophores:
1. Diploblastic with eight meridional rows of ciliated bands on external surface (Fig. 13.2) and these bands, called comb rows that help in swimming.
2. No polyp stage.
3. Ovoid or spherical and biradial symmetrical body.
4. Nematocysts are absent except Euchlora rubra.
5. Tentacles, if present, are two in number and bear short branches called pinnae and retractile into sheaths.
6. Presence of adhesive cells in the tentacular epidermis, called colloblasts or lasso cells (Fig. 13.3A), help in food capturing.
7. An aboral sense organ is present in the form of statocyst, acts as an organ of equilibrium.
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8. Presence of jelly-like mesoglea with muscle cells, of mesodermal origin. The source of formation of muscle cells during embryogenesis is still not clear.
9. Presence of pharynx or stomodaeum serves as a site of extracellular digestion. Digestion is both extracellular and intracellular.
10. Presence of four digestive canals with two anal pores. These pores are used for the discharge of the undigested wastes.
11. Nervous system is a sub-epidermal nerve-net.
12. There are no respiratory, circulatory or excretory systems.
13. Gonads are lodged in the same individual (hermaphrodite) on opposite sides of the meridional canals.
14. Fertilization is external.
15. Exclusively marine and are mostly planktonic.
5. Course of Development in Ctenophora:
Fertilized eggs (zygote) undergo total, biradial and determinate cleavage. Gastrulation takes place by epiboly or invagination. Gastrula usually develops directly into a free-swimming cydippid larva closely resembles the adult ctenophore.
6. Scheme of the Classification of Ctenophora:
The scheme of classification is based on the plan outlined by L. H. Hyman (1940).
The phylum Ctenophora is divided into two classes:
(i) Tentaculata (Micropharyngea) and
(ii) Nuda (Macropharyngea).
Classification with Characters:
Class 1.: Tentaculata or Micropharyngea:
The members of this class possess tentacles which are usually two in number.
The class is divided into four orders:
Order 1. Cydippida (Cydippidea or Cydippea):
The body is round or oval in form. Two branched tentacles retractile into sheaths are present. The meridional canals are un-branched.
Examples:
Hormiphora and Pleurobrachia, Lampetia, Callianira.
Order 2. Lobata:
The bases of the two principal tentacles are devoid of sheath and numerous non-retractile lateral tentacles are present. Two very large oral lobes are present.
Examples:
Deiopea, Bolina, Leucothea.
Order 3. Cestida (Cestoidea or Cestidea):
The body is extremely compressed in the sagittal plane to form a band-like appearance. Four of the eight meridional comb-plates are rudimentary. The two main tentacles are reduced.
Examples:
Cestus, Velamen.
Order 4. Platyctenea (Platyctenida):
The body is flattened and is modified for creeping movement. The comb-plates are restricted only in the larval stage.
Examples:
Ctenoplana, Tjalfiella, Coeloplana.
Class 2.: Nuda or Macropharyngea:
The representative of this class lacks tentacles. This class includes only one order.
Order 1. Beroida:
The mouth is very wide and the gullet occupies the greater portion of the interior of the body. The meridional canals are produced into a complex system of anastomosing branches. The sole genus of the order is Beroe (Fig. 13.5). Beroe is available in all seas and measures about 20 cm in height.
7. Affinities of Ctenophora:
(A) Interphylar Relationship:
Relationship with Sponges:
Many authors tried to establish the sponges as to be closely related to Ctenophora.
They put forward the following points to support their contentions:
1. The large central cavity and the osculum of sponges correspond to the coelenteron and mouth of Ctenophora respectively.
2. Absence of well-formed mesoderm in both,
3. Simpler organisation in both.
But closer examination reveals that these two groups are quite widely apart.
The points of differences are:
1. Developmentally the osculum of sponges does in no way correspond to the mouth of Ctenophora.
2. Presence of inhalant pores and peculiar collar cells in sponges are lacking in Ctenophora.
3. Absence of colloblasts in sponges.
4. Absence of specialised nervous and sensory structures in sponges.
Remark:
The above differences are sufficient to separate the two groups from each other, though the above relationship is emphasised by Leuckart and some other authors.
Relationship with Platyhelminthes:
Similarities between Polycladida (Platyhelminthes) and Ctenophora:
The idea that Ctenophora gave rise to certain bilateria (Polyclad) has been supported by many zoologists. Platyctenea has been considered to be a connecting link between Ctenophora and the bilateria.
1. Flat compressed body having a movement by creeping-like on the sole of the foot.
2. General ciliation of the body.
3. The dorsal polar nerve of Turbellaria can be compared with the statocyst of Ctenophora.
4. Origin of the so-called mesoderm is more or less similar.
5. Primary locomotor organs in the larva (Muller’s larva) consist of eight ciliated ridges of ectoderm which can be compared with the ctenophoran meridional comb-plates.
6. Dermal musculature well developed.
7. Branched gastro-vascular cavity.
8. Ctenophora exhibits both radial as well as bilateral symmetries.
9. Development of the two groups has a close resemblance.
Dissimilarities:
1. Polyclads have a highly developed brain.
2. Numerous eyes in polyclads.
3. No definite ciliary plates in Turbellaria.
4. Tentacles generally absent in Turbellaria.
5. Presence of flame cells in polyclads.
6. Ctenophores are radially symmetrical.
7. Complex reproductive system with a muscular penis in polyclads.
8. Embryological details vary in these two groups.
Remarks:
The view that the primitive bilateria have evolved through Platyctenea has not been accepted. Because a thorough examination of the platyctenea reveals that it is a ctenophore which has become extensively modified for sessile habits. It can further be suggested that platyctenea is a tissue-grade diploblastic animal, whereas polyclad is an organ-grade triploblastic form.
Again Lang (1881) advocated that the ctenophores led directly to the polyclads and that the Platyctenea bridge the gap between the Coelenterata and Platyhelminthes. Hadzi (1944) and de Beer (1954 & ’58) hold that Ctenophora arose from Polycladida by neoteny.
Relationship with Nemertines:
Similarities:
The larval form of Nemertine (Pilidium larva) shows some similarities with ctenophores.
1. Locomotion is performed by lobed bands of cilia in Pilidium larva.
2. The aboral end on the body contains a cup-like sense organ.
3. Radial structure without coelom.
4. Gonad ectodermal.
Dissimilarities:
1. Ctenophores lack mesoderm proper.
2. Paired tentacles with basal musculature are present in Ctenophora, absent in Nemertines.
3. The aboral sense organ in both differs widely because aboral calcareous sense organ in Ctenophora but absent in Nemertines.
Remarks:
The similarities may best be explained as to be due to distant convergence and perhaps with no phylogenetic significance.
(B) Intraphylar Relationship:
Similarities with Cnidaria:
The Ctenophora bear many characters of the cnidarians, but it differs considerably from the other members of the phylum Cnidaria.
Following features include them within phylum Cnidaria:
Cnidarian features:
1. Presence of basic radial symmetry.
2. Lack of coelom.
3. Diploblastic body wall. The mesodermal tissue is not distinct during embryogenesis.
4. Presence of gelatinous mesoglea.
5. Presence of ramified coelenteron.
6. Presence of diffused sub-epidermal nerve network.
7. Presence of statocyst as sense organ.
8. Absence of organ systems.
9. Arrangement of the parts of the body along an oral-aboral axis.
10. Presence of tentacles.
11. Gastro-dermal origin of gonads,
12. Presence of lasso cells in ctenophores similar to the nematoblasts of Cnidaria.
13. The body structure of ctenophores is superficially similar to medusae of cnidarians.
Dissimilarities:
The Ctenophora differs widely from the Cnidaria by the possession of the following features:
1. Possession of meridional combplates.
2. The symmetry of ctenophore is biradial rather than radial.
3. Lack of nematocysts.
4. Presence of adhesive colloblasts or lasso cells.
5. Presence of aboral sense organs.
6. Presence of only two tentacles in some ctenophores.
7. Higher and complicated organization of the digestive system.
8. Direct development of muscle cells from the mesenchyme.
9. Retention of multi-ciliated cilia as locomotor organs in adult ctenophores but monociliated cells in cnidarians.
10. Determinate (mosaic) type of development in Ctenophora but indeterminate type of development in cnidarians.
11. Gastrulation by epiboly or invagination in Ctenophora but delamination, or invagination in cnidarians.
12. ‘Cydippid’ is the common developmental stage in Ctenophora but Planula in cnidarians.
Remarks:
The similarities between the body architecture of ctenophores and the medusae of cnidarians may be due to convergence rather than a close evolutionary relationship (Ruppert and Barnes, 1994).
Relationship with Hydrozoa:
With Ctenaria (Anthomedusa):
Similar features:
1. Presence of two tentacles, situated at opposite per radii, each is provided with a deep pouch at its base, resembling closely the tentacular sheath of Hormiphora.
2. Presence of eight radial canals formed by the bifurcation of four inter-radial pouches of the stomach.
3. The subumbrellar cavity of the Ctenaria can be homologised with the stomodaeum of Hormiphora.
Dissimilar features:
1. The gullet of Ctenophora is ectodermal in origin.
2. The tentacles of Ctenaria have no muscular base.
3. The development of gonad is different. The gonads develop from manubrium in Ctenaria but in Ctenophora gonads develop from meridional canals.
4. Absence of the characteristic aboral sense organ in Ctenaria.
With Hydroctena (Narcomedusa):
Hydroctena (Fig. 13.6) a trachyline medusa, shows close resemblances with Ctenophora.
The resemblances are:
1. Possession of two tentacles with sheath located between the margin and apex of the bell.
2. Presence of aboral sense organ in Hydroctena can be compared with that of Ctenophora.
Remarks:
But the presence of swimming-plates in Ctenophora and the presence of velum in Hydroctena remain as important differences to visualise a close relation among them.
Relationship with Actinozoa:
Actinozoa exhibits some resemblances with the ctenophores.
Similarities:
1. Transverse section of the embryos in both of them are similar.
2. The stomodaeum can be homologised in both groups.
3. The gonads in both are endodermal in origin.
4. Mesoglea is cellular in both.
Dissimilarities:
1. Presence of aboral sense organ in ctenophora.
2. Presence of hollow tentacles in Actinozoa.
Remarks:
Due to the presence of vast structural diversities between the two groups, no relationship can be advocated though it was greatly emphasised by T. H. Huxley.
Relationship with Scyphozoa:
Similarities:
The scyphozoans have been looked upon as to be related to Ctenophora.
Because in both of them the following features are common:
1. The stomodaeum is similar.
2. Gametes are endodermal in origin.
3. Coelenteron is four-lobed.
Dissimilarities:
1. Presence of four oral arms.
2. Numerous marginal tentacles.
3. Absence of meridional comb-plates in Scyphozoa.
Remarks:
By the above evidences, it is not possible to connect both the groups together.
Discussion:
In search of affinity of ctenophores we find certain points of similarities here and there within the groups of cnidarians as a whole, but such similarities are not in any way enough to draw the cnidarian affinities of ctenophores.
8. Ancestry of Ctenophores:
Nothing is definitely known about the ancestry of the ctenophores. It is generally believed that they arose from a basic stock which gave rise to the cnidarians, thus there are certain resemblances and certain differences between the ctenophores and cnidarians.
Different Views regarding Systematic Position:
Sedgwick (1900) grouped the ctenophores with cnidaria under the same phylum Coelenterata and gave them a subphylum status.
Hyman (1940) stated that the ctenophores are a “blind offshoot” from the trachyline stem that attained a considerable grade of differentiation without leading to any higher forms. She, however, gave the ctenophores a separate phylar status and abolished the name “Coelenterata”.
Marshall and Williams (1972) also placed the ctenophores under phylum “Coelenterata” as a subphylum Acnidaria.
Barrington (1979) has stated that Ctenophores and Cnidarians may have had a common origin and the production of nematocysts by one ctenophore species, Euchlora rubra, possibly justifies this view and it is also said that there is nothing in the ultrastructure of the two groups to indicate a particularly close relationship.
Ruppert and Barnes (1994) stated that the triploblasic origin of their tissues and determinate cleavage do not consider ctenophores as cnidarians. They placed the ctenophores under a separate phylum Ctenophora.
Phylogenetic Status:
The proper status of Ctenophora still remains uncertain and offers a problem for further studies. It is now more or less universally accepted that the group separated out very early from the “trachyline stem” at the time when other groups of the cnidarians originated. It cannot, however, be denied that the Ctenophora presents certain structural features which resemble more to the forms with bilateral symmentry.
Considering all the features of ctenophores, it has now been proposed to treat these coelenterate- like animals under a separate phylum. As regards the relationships of different groups of Ctenophora, it can be suggested that both Cestidea and Lobata have been derived from Cydippid-like forms because during development both of them pass through a stage closely resembling the Cydippidea.
Beroidea is a highly organised form which may have arisen from some tentaculate form. Regarding Platyctenea whether they are primitive forms or specially modified is a question yet to be answered.
