Phylum Cnidaria

Dr. Enamul HaqueAsstt. ProfessorKrishnagar Govt. CollegeKrishnagar, NadiaWest BengalPhylum CnidariaCnidarians are diploblastic Metazoa at a tissue grade of construction. They possess primary radial symmetry, tentacles, stinging or adhesive structures called cnidae, an entodermally derived, incomplete gastrovascular cavity as their only body cavity, and a middle layer (called mesenchyme, or mesoglea) derived primarily from ectoderm. They lack cephalization, a centralized nervous system, and discrete respiratory, circulatory, and excretory organs . This basic bauplan is retained in both the polypoid and medusoid forms.Biological Contributions1. Both phyla have developed two well-defined germ layers, ectoderm and endoderm; a third, or mesodermal, layer, which is derived embryologically from ectoderm, is present in some. The body plan is saclike, and the body wall is composed of two distinct layers, epidermis and gastrodermis, derived from ectoderm and endoderm, respectively. The gelatinous matrix, mesoglea, between these layers may be structureless, may contain a few cells and fibers, or may be composed largely of mesodermal connective tissue and muscle fibers.2. An internal body cavity, the gastrovascular cavity, is lined by gastrodermis and has a single opening, the mouth, which also serves as the anus.3. Extracellular digestion occurs in the gastrovascular cavity, an intracellular digestion takes place in gastrodermal cells. Extracellular digestion allows ingestion of larger food particles.4. Most radiates have tentacles, or extensible projections around the oral end, that aid in capturing food.5. Radiates are the simplest animals to possess true nerve cells (protoneurons), but nerves are arranged as a nerve net, with no central nervous system6. Radiates are the simplest animals to possess sense organs,which include well-developed statocysts (organs of equilibrium) and ocelli (photosensitive organs).7. Locomotion in free-moving forms is achieved by either muscular contractions (cnidarians) or ciliary comb plates (ctenophores).However, both groups are still better adapted to floating or being carried by currents than to strong swimming.8. Polymorphism in cnidarians has widened their ecological possibilities. In many species the presence of both a polyp (sessile and attached) stage and a medusa (free-swimming) stage permits occupation of a benthic (bottom) and a pelagic (open-water) habitat by the same species. Polymorphism also widens the possibilities of structural complexity.9. Some unique features are found in these phyla, such as nematocysts (stinging organelles) in cnidarians and colloblasts (adhesive organelles) and ciliary comb plates in ctenophoresGeneral CharactersSome unique characteristicsNematocystsAlternation of generations (polyp and medusa body forms)Planula larvaMesoglea

1. Diploblastic Metazoa with ectoderm and entoderm separated by a (primarily) ectodermally derived acellular mesoglea or partly cellular mesenchyme2. Possess primary radial symmetry, often modified as biradial, quadriradial, or other form; theprimary body axis is oralaboral3. Possess unique stinging or adhesive structures called cnidae; each cnida resides in and is produced by one cell, a cnidocyte. The most common cnidae are called nematocysts4. Musculature formed largely of myoepithelial cells (= epitheliomuscular cells), derived from ectoderm and entoderm (adult epidermis and gastrodermis)5. Exhibit alternation of asexual polypoid and sexual medusoid generations; but there are many variations on this basic theme6. The entodermally derived gastrovascular cavity (coelenteron) is the only body cavity. The coelenteron is saclike, partitioned, or branched, but has only a single opening, which serves as both mouth and anus7. Without a head, centralized nervous system, or discrete gas exchange, excretory, or circulatory structures8. Nervous system is a simple nerve net(s), composed of naked and largely nonpolar neurons9. Typically have planula larvae (ciliated, motile, gastrula larvae)Class:- Hydrozoa

Characters:-Individuals are either exclusively polypoid or medusoid or constituted by both forms in the life cycle.(ii) Mesoglea acellular.(iii)Cnidocytes are confined to the epidermal layer. (iv)Gonads are usually ectodermal in origin.

e.g. Hydra , Obelia , Gonionemus, Physalia

Hydra , Obelia , Gonionemus, Physalia

Class:-Scyphozoa

Characters:- (i) Medusoid form is dominant in the life cycle, polypoid forms is very insufficant. (ii) Mesoglea is cellular.(iii)Gonads are endodermal. (iv)The endodermal gastric tentacles are present. Examples:- Aurelia; Pilema; Pericolpa;

Fig :- Aurelia Pilema

Class:-Anthozoa

Characters:-Exist only in the polypoid form.Gonads are endodermal.Stomodium strongly developed Extending between the stomodaeum & the body wall there are mesenteries.

Divided into two subclasses-SUBCLASS -ALCYONARIA (OCTOCORALLIA )SUBCLASS ZOANTHARIA (HEXACORALLIA)

SUBCLASS -ALCYONARIA (OCTOCORALLIA )

1. Polyp with eight pinnate tentacles and eight septa.2. Almost entirely colonial, and polyp usually connected by conenenchyme. e.g. Gorgonia (Sea fan), Coralium (precious red coral), Stylatula(Sea pen) , Tubipora , Pennatulla

Sea fan

SUBCLASS ZOANTHARIA (HEXACORALLIA)

CharactersPolyps with more than eight tentacles and septa, typically in the cycle of 12.Tentacles usually pinnate.Soltary or coloniale.g . Metridium