General Characteristics and Classification of Phylum

Discipline course -1 Semester -1 Paper – Lesson- General Characteristics and Classification of Phylum Cnidaria Lesson Developer: Sarita Kumar College /Department: Zoology, Acharya Narendra Dev College University of Delhi

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General Characteristics and Classification of Phylum Cnidaria

Table of Contents

• Introduction • Habit and Habitat • Body Organization • Absence of Coelom • Morphological Forms • • Medusa • Histology of Body wall • Epidermis • Gastrodermis • Mesoglea • Movement and Locomotion • Digestive System and Nutrition • Gas Exchange • Excretion and Osmoregulation • Nervous System • Sense Organs • Reproduction • Asexual Reproduction • Sexual Reproduction • Classification of Phylum Cnidaria • Summary

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General Characteristics and Classification of Phylum Cnidaria

• Exercise/Practice • Glossary • References/Bibliography/Further Reading

Introduction

Cnidaria is one of the most common phyla of , with over 11,000 different described species. The word ‘Cnidaria’ comes from the Greek word "Cnidos" Which means stinging nettle. Therefore the animals comprising Phylum Cnidaria are commonly called nettle animals or stinging animals. These are widely distributed animals found throughout the world which include hydras, sea fur, , sea anemones and .

Value Addition: Interesting to Know!! Heading Text: Discovery of Phylum Cnidaria

Body Text: The term Phylum Coelenterata was introduced by a German zoologist, Karl Georg Friedrich (1822–1898) who split Radiata into two phyla: Coelenterata and Echinodermata. Later, Berthold Hatschek (1854–1941), an Austrian zoologist based on his embryological and morphological studies of split Coelenterata into three phyla: Spongiaria, Cnidaria and . Hatschek in his paper wrote...”I advice the scientific and user communities (and the editors of the Zoological Record) to abandon the name Coelenterata altogether, not because this is right but because it has evidently been beaten by Cnidaria.”

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General Characteristics and Classification of Phylum Cnidaria

(a) (b) Fig. 1: (a) Karl Georg Friedrich Rudolf Leuckart, (b) Berthold Hatschek Source: http://www.repository.naturalis.nl/document/46835; http://en.wikipedia.org/wiki/Rudolf_Leuckart http://en.wikipedia.org/wiki/Berthold_Hatschek

Habit and Habitat

Almost 99% of the cnidarians are marine forms. These are distributed throughout the world.

• Most of them are situated on the shore of the sea and are littoral. • Some of these marine forms are active swimmers and are pelagic in habitat • Only a few are found in deep sea. The deep sea forms are often luminescent and emit light. • Freshwater cnidarians are very few. The most common example of fresh water cnidarians is Hydra.

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General Characteristics and Classification of Phylum Cnidaria

Fig. 2: Collection Sites: world map showing specimen collection locations for Cnidaria Source: http://eol.org/pages/1745/details

Value Addition: Interesting to know!! Heading Text: Bioluminescent Jellyfish

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General Characteristics and Classification of Phylum Cnidaria

Body Text: Pelagia noctiluca is a bioluminescent jellyfish with the ability to glow in the dark. In Latin, Pelagia means "of the sea", nocti stands for night and luca means light.

This species of jellyfish commonly known as the mauve stinger is widely distributed in all warm and temperate waters of the world's oceans, including the Mediterranean Sea, and Atlantic Ocean. It is also found in the Pacific Ocean, with sightings in warm waters off Hawaii, southern California and Mexico, as well as other Pacific locations. This is typically an offshore species, although sometimes it is washed near the coastlines and may be stranded in great numbers on beaches. The color varies worldwide, and in addition to pink or mauve, it is sometimes shades of golden yellow to tan

As its name implies P. noctiluca has the ability to bioluminescence, or produce light. Light is given off in the form of flashes when the medusa is stimulated by turbulence created by a ship’s motion or by waves. This flashing is only of relatively short duration and gradually fades.

Fig. 3: Pelagia noctiluca medusa near Sicily in the western Mediterranean Sea Source: http://en.wikipedia.org/wiki/Pelagia_noctiluca

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General Characteristics and Classification of Phylum Cnidaria

Body Organization

Cnidarians are diploblastic, radially symmetrical animals with tissue level of body organization. In few cnidarians, symmetry is biradial because of the elongation of mouth. Because of the symmetry, Cnidaria were earlier kept under Kingdom Radiata, along with ctenophores.

Value Addition: Did You Know?? Heading Text: Largest and Smallest Cnidarians

Body Text: Largest: The largest noncolonial Cnidarian is the Lion’s Mane Jellyfish, Cyanea arctica, with a bell diameter of up to 5.6 feet (1.7 meters) and a tentacle length of 120 feet (36.6 meters). The largest colonial Cnidarian, that is not a , is the Bubblegum Coral, that can reach heights of 9.8 feet (3 meters)

Smallest: The smallest Cnidarian is the tiny hydra, Psammohydra nanna, with a height of only 0.02 inches (0.4 mm).

(a) (b)

Fig. 4: (a) Lion’s Mane Jellyfish; (b) Hydra

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General Characteristics and Classification of Phylum Cnidaria

Source: http://www.storyofsize.com/corals-jellyfish/

Fig. 5: Radial Symmetry in Cnidarians Source: http://cnidaria2.wikispaces.com/ An important feature of cnidarians is the presence of a gut cavity lined by endoderm. It is called gastrovascular cavity or coelenteron. It has a dual function of digestion (gastro) and transportation (vascular) of food. The cavity open to the outside through an opening called mouth which is surrounded by a circle of tentacles. Absence of coelom

Cnidarians are acoelomate organisms. They do not have any body cavity or any fluid-filled space in their bodies. The body wall of cnidarians consists of two cellular layers. The outer epithelial layer is called epidermis and the inner epithelial layer is called gastrodermis. A thin/thick, non-cellular jelly-like layer, mesoglea is present in between two cellular layers. The water circulates through gastrovascular cavity and acts as hydrostatic skeleton.

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General Characteristics and Classification of Phylum Cnidaria

Fig. 6: Cross-section of an adult Jelly fish showing different layers of the body wall and absence of coelom Source: http://sargentanimalsystems.wikispaces.com/Respitory

Morphological Forms

Cnidarians exist in two different morphological body forms, polyp and medusa. a) Polyp • Polyp is a sessile form with elongated tubular body which is attached to a substratum by its aboral end. • The free oral end has a circular mouth in the center surrounded by tentacles. • Mesoglea if often a thin layer but may vary in thickness in different organisms. • Polyp can further exist in different forms for different functions; gastrozooids for feeding; dactylozooids and tentaculozooids for protection, gonozooids for reproduction, etc. • It reproduces asexually by budding and thus is devoid of gonads. b) Medusa • Medusa is a free-swimming form which exhibits umbrella-like or bell-shaped body. • It bears a quandrangular mouth on a central tubular projection, called manubrium. • Mesoglea is often thick and bulky. Institute of Lifelong Learning, University of Delhi

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General Characteristics and Classification of Phylum Cnidaria

• Like polyps, medusa also exists in different forms for carrying out different functions. For example, gonophores for reproduction, pneumatophores for floating, nectophores for swimming, etc. • Medusa bears gonads and reproduces sexually by formation of gametes. Male and female medusa are separate.

Fig. 7: Outline structure of polyp and medusa Source: http://cnidaria2.wikispaces.com/

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General Characteristics and Classification of Phylum Cnidaria

Fig. 8: Detailed structure of polyp and medusa Source: http://doralbio8.wikispaces.com/cnidarians

The phenomenon of existence of cnidarians as different individuals which differ in forms and functions is called polymorphism.

Histology of Body Wall

(A) Epidermis The epidermis of cnidarians consists of five types of cells a) Epithelio-muscular cells Institute of Lifelong Learning, University of Delhi

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General Characteristics and Classification of Phylum Cnidaria

• These are the columnar cell which rest against the mesoglea and are expanded distally to form the epidermal surface. • These are the most common cells forming the bulk of the epidermis. • The cells are nucleated and contain all typical cell organelles. • The base of each cell contains 2 to more extensions called muscle tails containing contractile myofibrils. These run parallel to the longitudinal axis of the body and collectively form a contractile longitudinal layer. • The muscle tails may or may not be embedded into the mesoglea. b) Interstitial cells • These are located between the basal ends of epithelio-muscular cells. • These are small and rounded cells with large nuclei. • Interstitial cells are undifferentiated embryonic cells which are totipotent in nature. Thus, they are capable of differentiating to any other kind of cell required by the organism. • They play significant role in the regeneration, growth and reproduction of the organism. c) • Cnidocytes are the characteristic cells of cnidarians which are commonly called stinging cells or nettle cells. • These are located throughout the epidermis, especially on the tentacles forming batteries of nematocysts. • Each is a rounded or ovoid cell containing a basal nucleus. • One end of the cell contains a short, stiff and bristle-like process, called cnidocil. • The cell contains a capsule, called cnidocyst or nematocyst which is filled with a proteinaceous poisonous fluid called hypnotoxin. • The capsule also contains a long, hollow thread tube with a broad base, present at the top of capsule. • The base of the thread tube has three inward-directed large spines, called barbs; and three spiral rows of tiny spines called barbules. • The base of the thread tube is covered by a lid, operculum. • The cnidocysts can be discharged under the influence of mechanical or chemical stimuli received by cnidocil. The stimulus increase the permeability of the capsule leading to sudden rush of water inside. It increases the osmotic pressure within

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General Characteristics and Classification of Phylum Cnidaria the capsule causing the opening of the operculum and eversion of the thread tube. The thread tube once discharged cannot be withdrawn and thus, nematocysts cannot be used again. Different cnidarians have different kinds of nematocysts: a) Penetrant (Stenotele): These nematocysts have open and barbed thread tube which on discharge shoot outs and enters the body of the prey. The tube releases hypnotoxin and paralyses or kills the prey. b) Volvent (Desmonemes): They have short, barbless thread tube closed at the end. On discharge, the tube wraps around the body of prey and slows down its movement enabling its capture. c) Glutinants: These have straight, barbless tubes which secrete a sticky substance. They help in attachment of the organism, movement and prey capture. d) Spirocyst: It has a thin, single walled capsule containing a long, spirally coiled, unarmed tubule of uniform diameter.

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General Characteristics and Classification of Phylum Cnidaria

Fig. 9: Discharge of nematocyst in Cnidaria

Source: http://commons.wikimedia.org/wiki/File:Nematocyst_discharge.png

Value Addition: Video Heading Text: Structure and Discharge of Nematocysts

Body Text: This video describes the structure and discharge of nematocysts by a cnidarian. It is an interesting description which clarifies the concept of nematocyst, the peculiar feature of cnidarians.

Source: http://www.youtube.com/watch?v=BpKKGB-ivQo

d) Mucus-secreting cells These cells are similar to epithelio-muscular cells except that they have a large number of mucous bodies in their apical portion. They are especially abundant in the basal disc of hydrozoans. e) Sensory and nerve cells • Sensory cells are present throughout the epidermis but are particularly abundant on the tentacle. These are elongated cells which are present perpendicularly to the epidermal surface. The base of each cell gives rise to a number of neuron processes while the distal end is connected to the nerve cell. • Nerve cells are located at the base of the epidermis next to the mesoglea. These are parallel to the mesoglea and structurally similar to multipolar neurons. The nerve cells are in connection with each other through synapse resulting in the formation of a nerve net. (B) Gastrodermis The endodermis, also called gastrodermis, of cnidarians is quite similar to epidermis and is simpler. It mainly contains four finds of cells. a) Endothelio-muscular or nutritive cells • These are structurally similar to the epithelio-muscular cells except that muscle tails are arranged transversely. As a result, they collectively form a contractile circular layer. • The free end of each cell has a long flagellum which helps to keep the food in motion in gastrovascular cavity.

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General Characteristics and Classification of Phylum Cnidaria

• These are nutritive cells and are filled with nutritive particles after feeding. b) Enzymatic gland cells • These are interspersed among the nutritive cells. • As the name implies, they secrete digestive enzymes and release in the gastrovascular cavity for digestion of food. c) Mucous-secreting gland cells • These are abundant around the mouth and secrete a slimy fluid which serves as lubricant. • The secretion helps to trap the prey for ingestion. d) Interstitial cells These are similar to those as present in epidermis, except that they are few in number. e) Sensory and nerve cells These are also similar to those present in epidermis.

Value Addition: Do you know!! Heading Text: Symbiotic Algae in Cnidaria

Body Text: A few cnidarians exhibit symbiotic relationship with algae, Zoochlorellae and which are present in their gastrodermis. These algae provide food and, green or brown colour to the organism possessing these alga. For example, Hydra viridissima (also known as Hydra viridis and Chlorohydra viridissima), or the green hydra, is a species of freshwater hydroid polyp in the genus Hydra found widely dispersed in the northern temperate zone. It is a common organism found in still waters from early spring to late autumn. The characteristic green colour comes from cells of the unicellular alga Chlorella within the cells of the gastrodermis. Because of this, Hydra viridissima are generally less predacious than aposymbiotic hydra species.

Source: http://en.wikipedia.org/wiki/Hydra_viridis

(C) Mesoglea The mesoglea of cnidarians is a jelly-like acellular layer which varies in thickness in different organisms. It serves as a supporting layer to the epidermis and gastrodermis. Being jelly- like in consistency it also provides elasticity to the body. The mesoglea exhibits the following: Institute of Lifelong Learning, University of Delhi

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General Characteristics and Classification of Phylum Cnidaria a) Dense glycogen granules b) Two network of nerve fibres; one of epidermis and other of endodermis c) Contractile muscle fibrils, longitudinal muscle fibrils arising from epithelio-muscular cells and circular muscle fibrils arising from endothelio-muscular cells

Fig. 10: Body wall of Hydra sp.

Cross sections of two regions of the hydroid body. A: hypostome. B: pedal disk. C: detail of the nerve cells forming a net on the base of the epidermis and gastrodermis. cm: circular muscular layer formed by the contractile extensions of the nutritive muscle cells; cn: cnidocyte; eg: epithelio-gland cell; ep: epithelio-muscular cell; g: mucous and enzymatic gland cell; i: interstitial cell; lm: longitudinal muscle layer formed by the contractile extensions of the epithelio-muscular cells; n: nerve cell and ns: neurosensory cell. Source: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077520 Institute of Lifelong Learning, University of Delhi

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General Characteristics and Classification of Phylum Cnidaria

Movement and Locomotion

Different cnidarians move in different ways.

• The polyps are sessile forms and attached to the substratum. They generally do not move from place to place. • However, polyps exhibit certain movements due to water currents or because of contraction and extension of longitudinal and circular muscles. • A few cnidarians, such as Hydra and sea anemones can move slowly over rocks and sea beds in various ways; creeping, crawling, or by somersaulting. Fluid present in the gastrovascular cavity acts as hydrostatic skeleton. • Medusae are free swimming forms. They generally move by Jet propulsion method - The contraction and expansion of bell muscles alternatively closes and opens the bell. It forces water out of the cavity inside the bell and propels the body in upward direction. This kind of jet propulsion method is called hydro propulsion. • Thick mesoglea of medusae provides them buoyancy and helps in floating on the sea surface. • Medusae also float passively in water by the force of strong water currents and wind drifting them here and there. Value Addition: Video Heading Text: Locomotion in Jellyfish

Body Text: These videos show the jet hydro propulsion movement of a jellyfish. One video shows the movement with the help of a dye. The dye disperses in the water behind the jellyfish because of its jet-like movement. The other video depicts the 3D movement of jelly fish in upward direction. Source: http://vimeo.com/5826452, http://www.youtube.com/watch?v=8tp_YxqLtYs

Digestive System and Nutrition

Most of the cnidarians are carnivorous organisms. They primarily feed upon small crustaceans, such as Cyclops, Daphnia; tadpoles, insect larvae, etc. A few cnidarians, however are omnivoroes and some are filter feeders.

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General Characteristics and Classification of Phylum Cnidaria

• The food is captured by the cnidocytes present on the tentacles. • The tentacles discharge the nematocysts which penetrate the prey, release hypnotoxin in theor body and paralyze them. • The tentacles capture the prey and pull in the gastrovascular cavity of the organism through the mouth. • The mucus secreted by the mucous-secreting gland cells present in the gastrodermis help in the ingestion of the organism. • Digestion begins in the gastrovascular cavity, where the enzymatic gland cells secrete the proteolytic enzymes. • The beating of the flagella present on these cells help in the mixing of the enzyme with prey for efficient digestion. • The proteolytic enzymes act on the food and partially digest it. This is called extracellular (outside the cell) digestion. • Now, the nutritive muscle cells engulf the semi-digested and small food particles in their food vacuole . • Here digestion of fats and proteins continues with the help of enzymes and this is called intracellular (inside the cell) digestion. • The digested products of the food are distributed throughout the body by cell-to-cell diffusion. • Some of the gastrodermal cells even may detatch and move freely to distribute the digested food. • The gastrovascular cavity, thus, serves for both digestion and transportation of food. • The undigested food material is egested through the mouth of the organism.

Gas Exchange

Cnidarians lack a respiratory system and any respiratory organs. The gas exchange takes place by diffusion through the general body surface. As Cnidarians are aquatic animals, oxygen directly diffuses from the surrounding water into the epidermal cells and carbon dioxide is diffused out. The diffusion of gases can also take place during circulation of water in the gastrovascular cavity of polyp or medusa as there is a continuous influx of water. Oxygen diffuses in the gastrodermal cells while carbon dioxide diffuses out in water.

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General Characteristics and Classification of Phylum Cnidaria

A few Anthozoans pump water out of and into their gastrovascular cavity through the ciliated grooves on their tentacles without using their mouth. It helps the cnidarians to control the water pressure within their cavity without having to expel any indigested food. Excretion and Osmoregulation

Cnidarians do not have special excretory or osmoregulatory organs. Being aquatic forms, they excrete nitrogenous waste in the form of ammonia that diffuses through the body wall. Excess water is thrown out of the gastrovascular cavity through their mouth. Thus, mouth being the single opening functions as a contractile vacuole too.

Nervous System

The nervous system of cnidarians is quite primitive. It comprises of two nerve nets on each side of mesoglea, at the base of epidermis and gastrodermis. These networks are particularly concentrated around the hypostome, mouth and basal disc.

The nerve cells contain two or more processes. Through these processes, nerve cells are connected to sensory cells (receptor) on one side and to epithelio-muscular cells or endothelio-muscular cells (effector) on the other side. They constitute a neurosensory system where the stimulus is received by sensory cells which is transmitted to muscle cells through the nerve net.

The peculiar feactures of the nervous sytem of cnidarians are as follows:

a) Double nerve net, often one slow and other rapid conducting system b) Non-synaptic, diffuse and irregular nerve nets c) Directionally non-polarized neurons transmitting impulses in all directions d) Stimulus in part of body transmits impulses in whole body and brings reactions e) Absence of simple reflexes

Sense Organs

The polyps of cnidarains lack any sense organs. However, medusae of cnidarians being free swimming possess several kinds of sense organs. These are as follows:

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General Characteristics and Classification of Phylum Cnidaria

Statocyst: These are the characteristic sense organs of medusa which serves as the organs of balance and equilibrium. They also control the rhythmic movement of the medusa. In Aurelia, these are modified tentacles, and thus called tentaculocyst or rhopalia.

Ocelli: A few cnidarians possess pigmented cells which act as photoreceptors and are sensitive to light.

Olfactory pits: In some cnidarians, sensory olfactory pits are present in association with tentaculocysts.

Reproductive System

The cnidarians reproduce by both asexual and sexual means.

a) Asexual reproduction:

The cnidarians reproduce asexually by budding during polyp stage. During the process, a small protuberance, called bud develops as an evagination of the body wall. It contains the extension of the gastrovascular cavity of the organism. Gradually, a mouth and tentacles develop at the distal end of the bud. The bud detaches itself from the parent body and becomes an independent organism

Value Addition: Animation!!

Heading Text: Budding in Hydra

Body Text: This is an interesting animation which depicts the process of budding in Hydra. The animation starts with the formation of bud followed by growth of bud and then its detachment from the parent body to grow as the adult Hydra.

Source: http://www.youtube.com/watch?v=wfbhwq95Duc&feature=youtu.be

.

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General Characteristics and Classification of Phylum Cnidaria

Fig. 11: Process of budding in Hydra

Source: http://www.devbio.biology.gatech.edu/?page_id=484 b) Sexual reproduction:

The sexual reproduction in cnidaria comprises of a complex life cycle which involves both polyp and medusa stages.

• Male and female meduase are separate. They release gametes in seawater where fertilization takes place. • Cleavage is radial, complete and determinate and results in the formation of a hollow blastula, called coeloblastula or solid blastula, called stereoblastula. • The gastrulation leads to the formation of an elongated and ciliated planula larva. • Gradually larva settles down on the ground by attaching itself at the anterior end. The mouth opens at the posterior end and gastrovascular cavity develops. • The larva develops into an adult organism.

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General Characteristics and Classification of Phylum Cnidaria

In a few cnidarians, such as jelly fish, the larva attached on the ground is termed as scyphistoma larva which represents the only polypoid stage of the jelly fish. It undergoes a process of strobilation during which it splits horizontally into a series of discs placed one above the other. Each of these discs separate as a free swimming medusoid ephyra larva. It swims actively in the seawater and gradually develops into adult jelly fish.

Fig. 12: Sexual reproduction in a jelly fish

Source: http://cnx.org/content/m44664/latest/?collection=col11448/latest

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General Characteristics and Classification of Phylum Cnidaria

Value Addition: Video

Heading Text: Coral spawning

Body Text: How does an like coral that lives attached to the bottom (and can’t move around) create new colonies far away? Watch the video and learn how these animals release eggs into the water at the right time of the year to create new coral colonies.

http://www.blueworldtv.com/webisodes/watch/coral-spawning Source: http://www.blueworldtv.com/

Value Addition: Do You Know??

Heading Text: Risk Statement

Body Text: The larval stage of cnidarians can cause a condition known as seabather's eruption. This should not be confused with cercarial dermatitis, which is caused by certain schistosomatid trematode flatworms (e.g., Austrobilharzia variglandis) that normally use birds and mammals other than humans as their definitive hosts.

The areas of skin affected by seabather's eruption is generally under the garments worn by bathers and swimmers where the organisms are trapped after the person leaves the water. In contrast, cercarial dermatitis occurs on the exposed skin outside of close-fitting garments. Source: http://eol.org/pages/1745/details

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General Characteristics and Classification of Phylum Cnidaria

Classification of Phylum Cnidaria

Phylum Cnidaria is divided into three classes based on theie existence in different morphological forms. a) • These are solitary or colonial animals. • These organisms have both polyp and medusa stages in their life cycle. • Medusae are craspedote with edge produced inwards as true velum. • Gastrovascular cavity present but without stomodaeum and mesentries. • Mesoglea is very thin. A few examples are Hydra, Obelia, Millepora, Physalia and Sertularia

(a) (b)

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General Characteristics and Classification of Phylum Cnidaria

(c) Fig. 13: Hydrozoans, (a) Physalia physalis, (b) Millepora (c) Pink Hearted hydroids Source: http://commons.wikimedia.org/wiki/File:Portuguese_Man-O- War_%28Physalia_physalis%29.jpg; http://wikieducator.org/Department_of_Zoology_at_ANDC/Zoology_Museum/Museu m_specimens/Cnidaria/Millepora http://www.oceanicresearch.org/education/wonders/cnidarian.html b) Scyphozoa • These are solitary and marine forms. • Medusa stage is dominant in the life cycle. • Polyp stage may be absent or reduced as a larval stage (scyphistoma). • Medusae are acraspedote with pseudovelum. • Mesoglea is very thick and jelly-like.

A few examples are Aurelia, Pilema and Periphylla

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General Characteristics and Classification of Phylum Cnidaria

(a) (b) Fig. 14: (a)Aurelia aurita (b) Lion's Mane Jelly, a venomous Scyphozoan Source: http://commons.wikimedia.org/wiki/File:Aurelia-aurita2_hg.jpg http://www.oceanicresearch.org/education/wonders/cnidarian.html c) Anthozoa • These are solitary or colonial animals which exist in only polyp forms. • Medusa stage is absent in the life cycle of anthozoans. • Gastrovascular cavity is well developed with stomodaeum and mesentries beset with nematocysts and gonads. • They may be surrounded by calcareous skeleton forming corals. • Corals may reproduce by continous budding and form large masses, coral reefs.

A few examples are Tubipora, Metridium, Alcyonium and Gorgonia

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General Characteristics and Classification of Phylum Cnidaria

(a) (b)

(c)

Fig. 15: Anthozoans, (a) Tubipora musica, (b) Metridium (c) Tealia anemone

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General Characteristics and Classification of Phylum Cnidaria

Source: http://wikieducator.org/Department_of_Zoology_at_ANDC/Zoology_Museum/Museu m_specimens/Cnidaria/Tubipora http://wikieducator.org/Department_of_Zoology_at_ANDC/Zoology_Museum/Museu m_specimens/Cnidaria/Metridium http://www.oceanicresearch.org/education/wonders/cnidarian.html

Value Addition: Interesting fact

Heading Text: Fire coral

Body Text: This is fire coral. It has a potent sting that leaves an itchy rash on human skin. It is not technically a coral, but a kind of hydroid that encrusts other objects (including other corals).

Source: http://www.oceanicresearch.org/education/wonders/cnidarian.html

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General Characteristics and Classification of Phylum Cnidaria

Summary

• Phylum Cnidaria comprises of animals commonly called nettle animals or stinging animals. • Cnidarians are ubiquitous and are mainly found in marine environments. • These are diploblastic, radially symmetrical animals with tissue level of body organization. • Body has a central gut cavity lined by endoderm, called gastrovascular cavity or coelenteron which opens by a single opening, mouth. It has a dual function of digestion and transportation of food. • These are acoelomates with body wall consisting of two cellular layers; outer epidermis and inner gastrodermis with a jelly-like mesoglea present in between. • Body is covered by a resistant, inelastic and thick cuticle which is covered by a thin and lipoid epicuticle. • Cnidarians exist in two different morphological body forms; tubular, sessile and asexual polyp and umbrella-like, free-swimming and sexual medusa. • Epidermal layer has characteristic muscular cells, gland cells, cnidoblasts, interstitial cells, nerve cells, and sensory cells; whereas gastrodermis is similar in structure except the presence of cnidoblasts. • Cnidoblasts are the organs of offence and defence and their presence is the most characteristic feature of cnidarians. • Cnidarians exhibit different kinds of movement produced by contraction and expansion of the circular and longitudinal muscles. • Digestion is both extracellular and intracellular. • Circulatory and respiratory system are absent. • Nervous system is primitive and consists of a diffuse network of non-polarized nerve cells. • Sense organs are poorly developed and include tentaculocysts, olfactory pits and ocelli. • Reproduction is both by asexual and sexual means. • Asexual reproduction takes place by budding and sexual reproduction involves the external fertilization of gametes.

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General Characteristics and Classification of Phylum Cnidaria

• Cleavage is radial, determinate and development includes planula larva. • Phylum Cnidaria is divided into three classes, Hydrozoa, Scyphozoa and Anthozoa, based on the presence or absence of polyp and medusa in their life cycle.

INSERT CLASSIFICATION OF CNIDARIA.ppt https://docs.google.com/file/d/0B0Izh6GcIA_DdlBNUktZ VjBUZzg/edit

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General Characteristics and Classification of Phylum Cnidaria

Exercise/Practice

Q1. Differentiate between a) Craspedotes and Acraspedotes b) Polyp and Medusa c) Penetrants and Volvents

Q2. Name the following a) Larva of cnidarians b) Organs of equilibrium in medusa c) Gut cavity of polyps d) Totipotent cells in the body wall of cnidaria e) Organs of offence and defence in Hydra f) Excretory product of cnidarians g) Polypoid stage in the life cycle of jelly fish h) Poisonous fluid secreted by nematocysts

Q3. Answer the following questions in short. (a) Describe different kinds of nematocysts present in Cnidaria and explain their significance. (b) What is the significance of gastrovascular cavity present in a cnidarian? (c) Why are organisms belonging to Phylum Cnidaria called stinging animals? (d) List the various cells present in the body wall of cnidarians. Give at least one function of each cell. (e) How do cnidarians reproduce by asexual means?

Q4. Answer the following questions in detail. (a) Classify Phylum Cnidaria till classes and write the characteristic features of each class. Support your answer with suitable examples. (b) Discuss the structure and function of two morphological forms of a cnidarian.

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General Characteristics and Classification of Phylum Cnidaria

(c) Describe the structure and function of the body wall of a cnidarians in detail. (d) How do cnidarians reproduce? Discuss with the help of diagrams. (e) Write the distinguishing features of Phylum Cnidaria. Glossary

• Radial synmmetry: The symmetry in which an animal can be divided into two equal halves along any plane passing through the axis • Cnidocytes: The cells of organs and defence in cnidarians • Epidermis: Outer cellular epithelial layer of the body wall • Gastrodermis: Inner cellular epithelial layer of the body wall • Gastrovascular cavity: Central gut cavity lined by endoderm which has a dual function of digestion and transportation of food • Hydropropulsion: Propulsion of the medusa in upward direction by forcing water out of the body through alternate contraction and expansion of muscles • Hypnotoxin: The proteinaceous poisonous fluid filled in the nematocysts which is injected into the prey for its capture • Manubrium: A central tubular projection in the body of medusa which bears a quandrangular mouth • Medusa: Umbrella-like, free-swimming and sexual morphological form of cnidarians • Mesoglea: Thin/thick, non-cellular jelly-like layer present in between two cellular layers of body wall • Polyp: Tubular, sessile and asexual morphological form of cnidarians • Tentaculocysts: Modified tentacles that serve as the organs of balance and equilibrium • Totipotent cells: The undifferentiated embryonic cells which are capable of differentiating to any other kind of cell required by an organism

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General Characteristics and Classification of Phylum Cnidaria

References/ Bibliography/ Further Reading

Suggested Readings • Barnes, R.D. (1982). Zoology, V Edition. Holt Saunders International Edition. • Ichhpujani, R.L. and Bhatia, R. (2002). Medical Parasitology, III Edition. Jaypee Brothers Medical Publishers (P) Ltd., New Delhi. • Arora, D. R and Arora, B. (2001). Medical Parasitology. II Edition. CBS Publications and Distributors.

Useful Web Links

• http://www.bioimages.org.uk/html/r164789.htm • http://cnx.org/content/m44664/latest/?collection=col11448/latest • http://sargentanimalsystems.wikispaces.com/Respitory • http://eol.org/pages/1745/details • http://cnidaria2.wikispaces.com/ • http://doralbio8.wikispaces.com/cnidarians • http://wikieducator.org/Department_of_Zoology_at_ANDC/Zoology_Museum/Museu m_specimens/Cnidaria • http://www.plosone.org/article/info:doi/10.1371/journal.pone.0077520 • http://www.oceanicresearch.org/education/wonders/cnidarian.html

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