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Moodle Interface General Characteristics and Classification of Phylum Cnidaria Discipline course -1 Semester -1 Paper – Zoology Lesson- General Characteristics and Classification of Phylum Cnidaria Lesson Developer: Sarita Kumar College /Department: Zoology, Acharya Narendra Dev College University of Delhi Institute of Lifelong Learning, University of Delhi General Characteristics and Classification of Phylum Cnidaria Table of Contents • Introduction • Habit and Habitat • Body Organization • Absence of Coelom • Morphological Forms • Polyp • 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 Institute of Lifelong Learning, University of Delhi 1 General Characteristics and Classification of Phylum Cnidaria • Exercise/Practice • Glossary • References/Bibliography/Further Reading Introduction Cnidaria is one of the most common phyla of animals, 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, jellyfish, sea anemones and corals. 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 Rudolf Leuckart (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 invertebrates split Coelenterata into three phyla: Spongiaria, Cnidaria and Ctenophora. 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.” Institute of Lifelong Learning, University of Delhi 2 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. Institute of Lifelong Learning, University of Delhi 3 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 Institute of Lifelong Learning, University of Delhi 4 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, Red 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 Institute of Lifelong Learning, University of Delhi 5 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 coral reef, is the Bubblegum Coral, Paragorgia arborea 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 Institute of Lifelong Learning, University of Delhi 6 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. Institute of Lifelong Learning, University of Delhi 7 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 8 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/ Institute of Lifelong Learning, University of Delhi 9 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 10 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
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