CNIDARIANS: CHARACTERISTICS AND CLASSIFICATION OF PHYLUM COELENTERATA
In 1887 Leuckart coined the name Coelenterata. This Coelenterata included sponges and Ctenophores. In 1888 Coelenterata is divided into 3 phyla.
1) Spongiaria 2) Cnidaria and 3) Ctenophora.
Therefore, Cnidaria is the most suitable name for this phylum. Coelenterata was divided into Cnidaria (for coelenterates proper) and Acnidaria (for the ctenophores). Hyman regarded ctenophora as a separate phylum.
1) Huxley (1856) proposed the name Hydrozoa.
2) Hackle in 1891 proposed the name Scyphozoa.
3) Ehrenberg in 1833 proposed the name Anthozoa.
The coelenterates are radial symmetrical, tentacle bearing aquatic organisms.
- All are aquatic, mostly marine but a few are fresh water forms. Ex: Hydra.
- They may be solitary or colonial.
- They may be sedentary or free-swimming.
- Head and segmentation is absent.
- Tentacles encircle the mouth in one or more whorls. They are used for food capture, and defense.
- These are all diploblastic animals. They show ectoderm and endoderm. In between jelly like mesoglea is present.
- The cnidoblasts are important defensive and offensive cells. They are useful for food capture.
- Undifferentiated free interestitial cells are found among the epithelial cells.
- Mouth is present. Anus is absent.
- Mouth leads into a central cavity called 'Coelenteron. Hence the name Coelenterate'.
- Coelenterates are acoelomate. Because there is no true body cavity, or coelom.
- Respiratory, circulatory and excretory systems are absent.
- These are radial symmetrical animals.
- Nervous system is diffused type.
- Polyp is a nutritive zooid and fixed zooid. The medusa is a free swimming zooid and sexual zooid.
- Nutrition is intercellular and intracellular.
- Locomotion in medusa is by muscles.
- In Coelenterates Polymorphism tendency is seen in some examples.
- Asexual reproduction is by budding.
- Sexual reproduction takes place by the development of gonad and sex cells. Planula larva is seen.
COELENTERATE CLASSIFICATION (CNIDARIA):
Phylum coelenterata is divided into two subphyla.
1) Subphylum Cnidaria, 2) Subphylum Acnidaria. Now-a-days these two subphyla are regarded as two independent phyla
SUB PHYLUM: CNIDARIA AND CTENOPHORA:
According to Parkar and Haswell Phylum Cnidaria is divided into three classes.
CLASS I: HYDROZOA:
- Hydrozoa animals are multicellular, diploblastic animals.
- They show mouth opening, and anus is absent.
- They show both polyp and medusa forms. Medusa is a reproductive zooid. Polyp is a fixed stage.
- In medusa the gasto-vascular-system is transversed by canals. In medusa definite sense organs like statocyst, nervous system, and muscular system are well developed.
- Polymorphic tendency is well developed.
- Gonads are seen.
- Alternation of generations is seen in the life history of these animals.
- Velum is present on the medusa (Craspedote)
Order 1: Hydroidea: Solitary or colonial forms. Polyp well developed. Sense organs or medusa are statocysts.
Sub-Order 1. Anthomedusae. Ex: Hydra, Bougainvillea.
Sub-Order 2. Leptomedusae. Ex: Obelia.
Order 2: Trachylina: Fixed stage is absent. They are all mobile medusae. Marginal sense organs are modified tentacles.
Sub-Order 1. Trachymedusae Ex : Petasus.
Sub-Order 2. Narcomedusae. Ex : Polycolpa.
Order 3: Hydrocorallina: It includes coral like hydrozoans. CaCO3 skeleton is secreted by coenosarc. Polyps are dimorphic.
1) Millipora (Hydrozoans coral)
2) Stylaster (Hydrozoan corals)
Order 4: Chondrophora: It includes organisms with big floats. Ex: 1) Velella 2) Porpita.
Order 5: Pteromedusae: Pelagic hydrozoans. Ex: Tetraplatia.
Order 6: Siphonophora: They show highest polymorphic tendency. Ex: 1) Physalia (Poutugese-man-of-war) 2) Halistemma.
CLASS II: SCYPHOZOA:
- Represented by medusoid forms.
- Sense organs are tentaculocysts.
- Gastrovascular system shows stomach and 4 gastric pouches. In the gastric pouches gastric filaments are present.
- Velarium is present with endodermal canal (Acraspedote).
- Gonads are endodermal in origin
- Medusa arises by strobilisation.
Order 1: Stauromedusae: (Lucernarida) Sense organs absent. Medusa is pyramidal shaped. Sedentary. Ex: 1) Lucernaria 2) Haliclystus.
Order 2: Coronatae: The umbrella shows coronary grooves. 4 to 16 tentaclocysts are present. Ex: 1) Periphylla. 2) Nausithoe (It lives inside Porifera animals (sponges).
Order 3: Cubotnedusae: Medusa is cubical 4 perradial tentacalocysts are present. Free swimming. Ex: 1) Choropsaimum (free medusa) 2)Chatybdaea.
Order 4: Semeastomeae (or) Discomedusae: Most common medusae. Medusa is disc shaped, 4 perradial and 4 interradial tentaculocysts are present. Ex: 1) Aurelia (Jellyfish) 2) Rhopilema 3) Pelagia (Luminescent Jellyfish)
Order 5: Rhyzostomeae: Free swimming medusa. The oral arms are branched. Tentacles absent. 8 or more tentaculocysts are present. Ex: 1) Pilema 2) Rhizostoma.
CLASS III: ANTHOZOA:
- They exhibit only polyp forms.
- Medusa stage is absent.
- Mesentries are present, they bear nematocyst.
- Gonads are endodermal.
- Corals coral reefs are common.
This class is divided into 2 sub classes.
i) Sub class: Hexacorallia (or) Zoantharia.
ii) Sub class: Octocorallia (or) Alcyonaria.
Sub class: Hexacorallia: It contains tentacles and mesentries in multiples of six. It includes seven orders.
Order 1: Actinaria: (Sea anemones) Skeleton is absent. Ex: 1) Edwardsia 2) Adamsia
Order 2: Madreporaria: (True corals) Stony corals are present Polyps are small. Siphonoglyphs absent. Ex: 1) Meandrina (brain coral) 2) Fungia.
Order 3: Zoanthidea: Solitaryor colonial organisms. Polyps are united by basal stolons. Only ventral siphonoglyphis present. Ex: 1) Zoanthus.
Order 4: Antipatharia: Includes black corals. Two siphonoglyphs are present. Ex: Antipathes (Black coral)
Order 5: Ceriantharia: Solitary structure. Tentacles many, arranged into two whorls. Only single siphonoglyph occurs. Ex: Cerianthus.
Order 6: Corallimorpharia: Solitary or aggregate, anemone like polyps. Ex: Corynactis.
Order 7: Ptychodactaria: Includes animals which are anemone like polyps. Ex: Ptychodactis.
Sub class : Octocoralia (Alcyonaria): In these Anthozoan members the tentacles and mesentries are in multiples of eight. On the stomodium never more than one siphonoglyph will be present. It is ventral in position.
Order 1: Stolonifera: Polyps are connected by creeping stolon. Ex: Tubipora (orange pipe coral).
Order 2: Telestacea: The colonies contain simple or branched stem which bears lateral polyps. Ex: Telesto.
Order 3: Alcyonacea: These are soft corals. Polyps may be dimorphic. Ex: 1) Alcyonium (dead man's fingers).
Order 4: Coenothecalia: It includes a single genus. Ex: Heliopora (Blue coral)
Order 5: Gorgonacea: It is a compound tree like coral. Ex: 1) Gorgonia (seafan) 2) Corallium (red coral)
Order 6: Pennatulacea: These are elongated members. Emended in the mud, and sea bottom. Ex: Pennatula (Sea pen)
SUB PHYLUM: ACNIDARIA:
According to Parker and Haswell this sub phylum is treated as PHYLUM CTENOPHORA.
- These are biradially symmetrical organisms.
- They do not show nimatocysts.
- They show ciliary plates in 8 rows (comb plates in 8 rows).
- The mesoglea is traversed by muscle fibres.
- They may bear two tentacles. On tentacle Lasso cells are present.
- On the aboral pole an aboral sense organ is present, which is a modified lithocyst.
- There is no alternation of generations. This phylum is divided into two classes.
CLASS I: TENTACULATA:
Ctenophores without tentacles. This class is divided into four orders.
Order 1: Cydlppidea: Two retractile tentacles. Ex: Hormiphora
Order 2: Cestidea: The bases of the two principal tentacles are present. They are enclosed in sheaths. Ex: Cestus
Order 3: Lobata: Many lateral non-retractile tentacles are present. The bases of the two principal tentacles are present. Ex: Deiopea
Order 4: Platyctenea: They are creeping or sessile forms. They show a pair of retractile tentacles. Ex: Ctenoplana
CLASS II: NUDA:
Ctenophores without tentacles. This class includes only one order.
Order 1: Beroldea: Mouth is wide. Gullet occupies greater part of the body. Ex: Beroe
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