1. Introduction
Phylum Cnidaria includes aquatic, predominantly marine animals such as hydras, jellyfishes, sea anemones and corals. The name Cnidaria is derived from the Greek word knide, meaning nettle, referring to the characteristic stinging cells of the group. These animals are structurally more complex than sponges and show a distinct level of tissue organization. Cnidarians are important in zoology because they demonstrate radial symmetry, a diploblastic body plan, a gastrovascular cavity and specialized cells for prey capture and defence.
2. General Characteristics of Phylum Cnidaria
Cnidarians are multicellular aquatic animals. Most species are marine, while a few occur in freshwater.
They are generally solitary or colonial. Colonies may be formed by repeated asexual budding and may contain specialized individuals.
The body shows radial symmetry or modified radial symmetry. Structures are commonly arranged around a central oral and aboral axis.
They exhibit tissue level of organization. Cells are organized into functional tissues, although true organs and organ systems are absent.
They are diploblastic animals. The body develops from two primary germ layers, ectoderm and endoderm, with a noncellular or cellular mesoglea between them.
The body is usually sac like or cylindrical and has a single opening called the mouth.
The mouth generally opens into a gastrovascular cavity or coelenteron. A separate anus is absent in the typical condition.
Digestion is both extracellular and intracellular. Enzymatic digestion begins in the gastrovascular cavity and is completed within nutritive cells.
Tentacles are usually present around the mouth or oral region. They help in prey capture, defence and manipulation of food.
The diagnostic feature is the presence of cnidocytes, also called cnidoblasts, which contain specialized capsules known as nematocysts or cnidae.
Cnidocytes are especially abundant on the tentacles and oral region. Their discharge may immobilize prey or assist in defence.
The nervous system is primitive and consists mainly of a diffuse nerve net. A centralized brain is absent.
Respiratory and circulatory systems are absent. Gases and dissolved substances move mainly by diffusion.
A specialized excretory system is absent. Nitrogenous wastes diffuse through the body surface and gastrovascular cavity.
The mesoglea lies between the ectoderm and endoderm. It is relatively thin in polyp forms and may become thick and gelatinous in medusae.
Two principal body forms occur in the phylum, namely polyp and medusa. The polyp is generally cylindrical and attached, whereas the medusa is usually free swimming and umbrella shaped.
Many cnidarians show alternation between polyp and medusa forms during the life cycle. The phenomenon is commonly called metagenesis when sexual and asexual phases alternate.
Asexual reproduction commonly occurs by budding, especially in polyps and colonial forms.
Sexual reproduction is common. Gametes are usually produced by specialized tissues, and fertilization may be external or internal depending on the group.
Development commonly includes a ciliated planula larva. The planula eventually settles and develops into a polyp in many forms.
The phylum contains ecologically important reef building organisms. Corals contribute to the formation of coral reefs and provide habitats for many marine organisms.
3. Detailed Explanation of Important Characteristics
3.1 Habitat and Distribution
Cnidarians are primarily aquatic. Marine habitats contain the great majority of species, ranging from shallow coastal waters to deeper marine environments. Freshwater representatives are comparatively few, with Hydra being the best known example. Corals and many colonial forms are strongly associated with marine ecosystems.
3.2 Symmetry
Most cnidarians possess radial symmetry. The body can be divided into similar sectors by planes passing through the longitudinal axis. This arrangement is advantageous for animals that interact with their environment from several directions. Some members show variations from the typical radial condition.
3.3 Level of Organization
Cnidarians possess tissue level organization. Groups of cells are differentiated for functions such as protection, digestion, contraction, sensation and reproduction. The organization is more advanced than the cellular level found in Porifera, but it does not reach the organ level typical of higher animals.
3.4 Diploblastic Body Plan
The body wall develops from two primary germ layers. The outer ectoderm forms the epidermis and several specialized cell types. The inner endoderm, often termed gastrodermis, lines the gastrovascular cavity and contains cells involved in digestion. Between these layers is mesoglea, which varies greatly in thickness among different cnidarians.
3.5 Gastrovascular Cavity
The gastrovascular cavity is the central digestive chamber. The mouth serves as the entrance for food and also as the route through which undigested material is expelled. The cavity may be simple or extensively subdivided depending on the group. In many forms, it also helps distribute nutrients and gases within the body.
3.6 Cnidocytes and Nematocysts
Cnidocytes are specialized cells unique to cnidarians. Each cnidocyte contains a cnida, commonly represented by a nematocyst. A nematocyst consists of a capsule containing a coiled thread that can be rapidly discharged. Mechanical or chemical stimulation triggers discharge. The discharged structure may penetrate, entangle or adhere to the target. Cnidocytes are essential for prey capture and protection.
3.7 Polyp and Medusa
The polyp is generally tubular and attached by its aboral end. The mouth and tentacles are directed upward. The medusa is generally free swimming, umbrella shaped and has the mouth directed downward. Tentacles commonly occur around the margin. Different classes vary in the relative importance and structure of these two forms.
3.8 Nervous System
Cnidarians possess a diffuse nervous system consisting mainly of interconnected nerve cells. The nerve net coordinates muscular and sensory activities. More complex medusae may possess nerve rings, sensory structures and specialized balance organs. Nevertheless, a centralized brain is absent.
3.9 Reproduction and Development
Asexual reproduction commonly occurs by budding and may result in colony formation. Sexual reproduction involves the formation of gametes. The fertilized egg develops into a ciliated planula larva in many groups. The planula settles and gives rise to the next developmental stage.
4. Basis of Classification
Classification within Cnidaria is based mainly on the relative development of polyp and medusa stages, form and organization of the gastrovascular cavity, structure of the mesoglea, arrangement of tentacles, nature of reproductive structures, type of nematocysts, and whether the animals are solitary or colonial. For undergraduate study, the traditional classification into four major classes provides a useful framework.
5. Classification of Phylum Cnidaria up to Class
The following traditional classification is suitable for B Sc Zoology study. Modern phylogenetic classifications may differ in the placement and recognition of some groups, but the four class arrangement remains widely used in introductory zoology teaching.
Taxonomic Rank | Taxon | Principal Character | Representative Examples |
Phylum | Cnidaria | Cnidocytes, tissue level organization, diploblastic body | Hydra, Aurelia, Adamsia, Obelia |
Class | Hydrozoa | Polyp commonly prominent; medusa present in many forms; mesoglea generally not cellular | Hydra, Obelia, Physalia |
Class | Scyphozoa | Medusa is the dominant stage; polyp reduced and represented by scyphistoma | Aurelia, Rhizostoma |
Class | Cubozoa | Box shaped medusae with well developed sense organs and muscular activity | Carybdea, Chironex |
Class | Anthozoa | Only polyp stage; medusa absent; gastrovascular cavity divided by mesenteries | Metridium, Adamsia, Fungia, Madrepora |
5.1 Class Hydrozoa
Hydrozoans are mostly marine, although Hydra is a freshwater representative.
The polyp is commonly well developed and may be the dominant form. A medusa is present in many genera but is absent in some groups.
The mesoglea is usually relatively thin and generally noncellular in traditional descriptions.
The gastrovascular cavity is generally simple and lacks the elaborate radial canals characteristic of many scyphozoan medusae.
Many species are colonial and show division of labour among specialized zooids.
Asexual budding is common in polyps, while medusae or specialized reproductive structures may produce gametes.
Examples include Hydra, Obelia and Physalia.
5.2 Class Scyphozoa
Scyphozoans are predominantly marine and are commonly known as true jellyfishes.
The medusa is the dominant and conspicuous stage of the life cycle.
The polyp stage is small and is commonly called scyphistoma.
The medusa possesses a thick, gelatinous mesoglea and a well developed gastrovascular system.
Gonads occur in association with the gastrovascular system, and reproduction is primarily sexual in the medusa.
A ciliated planula develops from the fertilized egg, followed by the polyp stage in the typical life cycle.
Examples include Aurelia and Rhizostoma.
5.3 Class Cubozoa
Cubozoans are marine cnidarians commonly called box jellyfishes because the medusa has a roughly cubical or box shaped bell.
The medusa is the dominant stage and is an active swimmer.
The bell margin bears specialized structures associated with sensory perception and coordination.
Well developed eyes and sensory structures occur in many members.
The animals may possess powerful nematocysts, and some species are medically important because their stings can be severe.
The polyp stage is small and undergoes development into the medusa.
Examples include Carybdea and Chironex.
5.4 Class Anthozoa
Anthozoans are exclusively marine.
Only the polyp stage occurs. A medusa stage is completely absent.
The body is usually cylindrical with the mouth surrounded by tentacles.
The gastrovascular cavity is subdivided by radial partitions called mesenteries or septa.
The pharynx is a characteristic internal region connecting the mouth with the gastrovascular cavity.
Members may be solitary or colonial. Many corals secrete a skeleton of calcium carbonate.
Reproduction may be asexual or sexual, and a planula larva is commonly formed after sexual reproduction.
Examples include sea anemones such as Metridium and Adamsia, and corals such as Fungia and Madrepora.
6. Comparative Account of the Major Classes
Feature | Hydrozoa | Scyphozoa | Cubozoa | Anthozoa |
Main habitat | Mostly marine; Hydra freshwater | Marine | Marine | Marine |
Dominant form | Polyp commonly prominent | Medusa | Medusa | Polyp only |
Medusa | Present in many; absent in some | Well developed | Well developed | Absent |
Polyp | Usually well developed | Reduced | Small stage | Only body form |
Mesoglea | Usually thin | Thick and gelatinous | Thick | Relatively limited |
Gastrovascular cavity | Generally simple | Complex with canals | Complex | Divided by mesenteries |
Representative example | Hydra | Aurelia | Chironex | Metridium |
7. Important Terms
Cnidocyte: A specialized stinging cell characteristic of Cnidaria.
Cnida: The specialized capsule or organelle contained within a cnidocyte.
Nematocyst: A commonly encountered type of cnida containing a rapidly discharged thread.
Polyp: A generally cylindrical and attached body form with the mouth directed upward.
Medusa: A generally free swimming and umbrella shaped body form with the mouth directed downward.
Mesoglea: Material lying between the ectoderm and endoderm.
Gastrovascular cavity: The internal cavity responsible for digestion and distribution of nutrients.
Planula: A ciliated larval stage commonly produced after fertilization.
Metagenesis: Alternation of asexual polyp and sexual medusa phases in the life cycle of many cnidarians.
Mesentery: A radial partition that subdivides the gastrovascular cavity of anthozoans.
8. Examination Focus
Remember that cnidocytes are the defining cellular feature of Cnidaria.
Distinguish polyp and medusa by body orientation, mode of life and reproductive role.
Remember that Hydrozoa commonly emphasizes the polyp, Scyphozoa emphasizes the medusa, Cubozoa has a box shaped medusa, and Anthozoa lacks a medusa.
Learn the four major classes and at least two representative examples of each.
Understand the significance of the gastrovascular cavity and the absence of separate digestive, circulatory, respiratory and excretory organs.
For long answer questions, organize the general characteristics under habitat, symmetry, organization, body wall, digestion, cnidocytes, nervous system, reproduction and life cycle.
9. Multiple Choice Questions
1. The characteristic stinging cells of Cnidaria are called
A. Choanocytes
B. Cnidocytes
C. Flame cells
D. Colloblasts
Answer: B. Cnidocytes
2. The digestive cavity of cnidarians is called
A. Coelom
B. Pseudocoel
C. Gastrovascular cavity
D. Haemocoel
Answer: C. Gastrovascular cavity
3. Cnidarians are generally
A. Triploblastic
B. Diploblastic
C. Monoblastic
D. Acoelomate triploblastic
Answer: B. Diploblastic
4. The free swimming body form of many cnidarians is
A. Polyp
B. Medusa
C. Scyphistoma
D. Planula
Answer: B. Medusa
5. Which class has only the polyp form
A. Hydrozoa
B. Scyphozoa
C. Cubozoa
D. Anthozoa
Answer: D. Anthozoa
6. The dominant stage in Scyphozoa is
A. Polyp
B. Medusa
C. Planula
D. Larva
Answer: B. Medusa
7. Box jellyfishes belong to
A. Hydrozoa
B. Scyphozoa
C. Cubozoa
D. Anthozoa
Answer: C. Cubozoa
8. Hydra belongs to
A. Hydrozoa
B. Scyphozoa
C. Cubozoa
D. Anthozoa
Answer: A. Hydrozoa
9. Sea anemones belong to
A. Hydrozoa
B. Scyphozoa
C. Cubozoa
D. Anthozoa
Answer: D. Anthozoa
10. The common larva of many cnidarians is
A. Trochophore
B. Planula
C. Nauplius
D. Veliger
Answer: B. Planula
10. Question Bank
Very Short Answer Questions
What is a cnidocyte
What is a nematocyst
Define polyp
Define medusa
What is mesoglea
Name the larval stage commonly found in cnidarians
Name the class to which Hydra belongs
Name the class containing true jellyfishes
Name the class containing sea anemones and corals
What is the function of the gastrovascular cavity
Short Answer Questions
Describe the diploblastic organization of cnidarians.
Explain the structure and significance of cnidocytes.
Differentiate between polyp and medusa.
Write a short account of the gastrovascular cavity.
Describe reproduction in Cnidaria.
Write the important characteristics of Class Hydrozoa.
Write the important characteristics of Class Scyphozoa.
Write the important characteristics of Class Cubozoa.
Write the important characteristics of Class Anthozoa.
Long Answer Questions
Give a detailed account of the general characteristics of Phylum Cnidaria.
Discuss the organization of the cnidarian body with special reference to symmetry, germ layers, mesoglea and gastrovascular cavity.
Describe the different body forms of cnidarians and explain their significance in the life cycle.
Classify Phylum Cnidaria up to class and discuss the salient features and examples of each class.
Compare Hydrozoa, Scyphozoa, Cubozoa and Anthozoa on the basis of body forms, life cycle and major structural features.
11. References
Ruppert, E. E., Fox, R. S. and Barnes, R. D. Invertebrate Zoology: A Functional Evolutionary Approach. Cengage Learning.
Brusca, R. C., Moore, W. and Shuster, S. M. Invertebrates. Sinauer Associates.
Pechenik, J. A. Biology of the Invertebrates. McGraw Hill.
Jordan, E. L. and Verma, P. S. Invertebrate Zoology. S Chand Publishing.
Kotpal, R. L. Modern Textbook of Zoology: Invertebrates. Rastogi Publications.
Hickman, C. P., Roberts, L. S., Keen, S. L. and Larson, A. Integrated Principles of Zoology. McGraw Hill.
Quick Revision Box
Cnidaria in one view Aquatic animals | Mostly marine | Radial symmetry | Tissue level organization | Diploblastic | Mesoglea present | Gastrovascular cavity present | Single body opening | Cnidocytes present | Polyp and medusa forms | Diffuse nerve net | No specialized circulatory, respiratory or excretory systems | Planula larva common | Four major classes: Hydrozoa, Scyphozoa, Cubozoa and Anthozoa.
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