Phylum Cnidaria Characteristic and Classification

Phylum Cnidaria: Cnidaria derives from the Greek word “cnidos,” which means “stinging thread.”

Defination: The cnidaria may be defined as “diploblastic Metazoa with tissue grade of construction having nematocysts and a single gastrovascular cavity or the coelenteron.”

The phylum Cnidaria is a large group of aquatic invertebrates, with over 11,000 described species. They are mostly marine animals, but some species can be found in freshwater environments. Cnidarians are well-known for their stinging cells, called cnidocytes, which they use to capture prey and defend themselves. Their bodies consist of a non-living jelly-like substance called mesoglea, sandwiched between two layers of epithelium that are mostly one cell thick.

General Characteristric of Phylum Cnidaria

1. Cnidarian are Metazoa or multicellular animals with tissue grade of organization.
2. They are aquatic, mostly marine except few freshwater forms like Hydras.
3. They are sedentary or free-swimming and solitary or colonial.
4. Individuals are radially or biradially symmetrical with a central gastrovascular cavity communicating to the exterior by the mouth.
5. They are diploblastic animals; body wall consists of an outer layer of cells called ectoderm and inner layer of cells the endoderm cemented together by an intermediate layer of non-cellular gelatinous mesogloea.
6. Acoelomate animals because they do not possess a second body cavity, the coelom.
7. Short and slender tentacles encircle the mouth in one or more whorls.
8. The tentacles are provided with nematocysts; tentacles serve for food capture, its ingestion and for defence. These are also present on body layers, these are adhesive organs.
9. They exhibit the phenomenon of polymorphism with very few exceptions; the main types of zooids in polymorphic forms are polyps and medusa. Polyp is sessile and asexual zooid, while medusa is free-swimming and sexual zooid.
10. Skeleton, either exoskeleton or endoskeleton is of common occurrence.
11. They are usually carnivorous; digestion is extracellular as well as intracellular. Anus is not found.
12. Nervous system consists of one or more networks or nerve-cells and neurites located in the ectoderm and endoderm.
13. Respiratory, circulatory and excretory systems are wanting. 14. Reproduction is both by asexual and sexual methods.
14. Asexual reproduction occurs by budding and sexual reproduction by the formation of gametes.
15. A ciliated planula larva usually present in the life history.
16. The life history exhibits the phenomena of alternation of generations or metagenesis which the asexual polypoid, sessile generation alternates with sexual medusoid, free-swimming

Classification of Phylum Cnidaria

Cnidarians are classified into four main classes:

Class I- Anthozoa (Gr., anthos-flower; zoios animal)

1. This group includes sea anemones, corals, and sea pens. They are almost wholly sessile (non-moving) organisms.
2. They are exclusively polypoid.
3. Medusoid stage is altogether absent.
4. Body usually cylindrical with hexamerous, octomerous or polymerous biradial or radio- bilateral symmetry.
5. The oral end of the body is expanded radially into an oral disc bearing hollow tentacles surrounding the mouth in the centre.
6. The stomodaeum is present, often provided with one or more ciliated grooves the siphonoglyphs.
7. Gastrovascular cavity is divided into compartments by complete or incomplete septa or mesenteries.
8. Mesenteries bear nematocysts at their inner free edges.
9. Mesogloea contains fibrous connective tis- sue and amoeboid cells.
10. Skeleton either external or internal.
11. Exoskeleton is formed from calcium carbon- ate which often form massive corals.
12. Nervous system is in the form of typical nerve net without a concentrated central nervous system.
13. Gonads are endodermal, develop in the mesenteries.
14. The ripe sexual products are discharged into coelenteron.
15. Fertilization is external.
16. The fertilized egg develops into a planula larva, which after a short free life settles down and develops into an adult.

Examples: Tubipora, Telesto, Heliopora, Gorgonia, Corallium, Pennatula, Metridium, Admasia, Fungia, Favia, Madrepora, Zonathus, Antipathes Sea anemones etc.

Class II- Scyphozoa (Gr., skyphos = cup; zoios = animal)

1. These are the jellyfish, which are swimming forms.
2. Scyphozoa include large jelly-fishes or true medusae are exclusively marine.
3. Medusae are large, bell or umbrella-shaped, without true velum, free swimming or at- tached by an aboral stalk.
4. Marginal sense organs are tentaculocysts having endodermal statoliths.
5. Polypoid generation absent or represented by small polyp, the scyphistoma which gives rise to medusae by strobilization or trans- verse fission.
6. Gastrovascular system without stomodaeum with gastric filaments and may or may not be divided into four inter-radial pockets by septa.
7. Mesogloea is usually cellular.
8. Gonads are endodermal and the sex cells are discharged.

Examples: Lucernaria, Charybdaea, Pericolpa, Aurelia,Cyanea, Phyllorhiza punctata etc.

Cubozoa

1. These are also free-swimming jellyfish, but they have a cube-shaped body and four tentacles at each corner.
2. Box jellyfish are known for their potent venom, which can be deadly to humans.
3. Cubozoans have a complex visual system with four rudimentary eyes and a well-developed nervous system.

Examples: Carybdea branchi

Hydrozoa (Gr., hydra= water; zoios =animal)

1. A diverse group that includes both freshwater and marine cnidarians. It comprises sessile members (like Hydra) and colonial swimmers (such as the Portuguese man of war).
2. They exhibit tetramerous or polymerous ra dial symmetry.
3. Body wall consists of an outer ectoderm and an inner endoderm separated by non-cellular mesogloea.
4. Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament.
5. Skeleton or horny structure is horny peri- sarc in some forms, while coenosarc sec- retes a skeleton of calcium carbonate form- ing massive stony structure or coral in other forms.
6. They exhibit polymorphism. There are two main types of zooids, the polyp and medusa.
7. Medusa is provided with true muscular velum.
8. Many Hydrozoa exhibit alternation of generations.
9. Reproductive products or sex cells are usu- ally ectodermal in origin and discharged ex- ternally.
10. Cleavage is holoblastic, embryo ciliated planula.

Examples: Hydra, hydractinia, Tubalaria, Millepora, Obelia, Diphyes, Physalia physalis etc.