Animal Diversity- I (Non-Chordates) Phylum Cnidaria (Old name- Coelenterata) Dr. Sukanya Lal Zoology Department, Ramjas College, University of Delhi Delhi – 110 007 26th June 2007 Page 1 of 115 Chapter index I. Phylum Cnidaria (Coelenterata): General characters 1. Introduction 2. Morphology 3. Reproduction II. Classification of Phylum-Cnidaria (Coelenterata) 1. Introduction 2. Classification A. Class: Hydrozoa B. Class Scyphozoa C. Class: Cubomedusae D. Class: Anthozoa I Subclass: Alcyonaria (Octocorallia) II. Subclass: Zoantharia (Hexacorallia) III. Obelia geniculata 1. Habit and Habitat 2. Structure A. Polyp B. Blastostyle C. Medusa 3. Histological structure of zooids A. Epidermis B. Mesogloea C. Gastrodermis / Endodermis 4. Statocyst 5. Gonads 6. Locomotion 7. Feeding 8. Excretion 9. Respiration 10. Reproduction A. Asexual mode of reproduction B. Sexual mode of reproduction 11. Alternation of generation and Metagenesis 12. Polymorphism. A. Polyps B. Blastostyle C. Medusa Figures 1-10 Page 2 of 115 IV. Aurelia aurita (Jelly-fish) 1. Introduction 2. Habit and Habitat 3. Structure A. Oral arms and Mouth B. Radial canals C. Velarium D. Sub-genital pits E. Gonads 4. Histological structure A. Epidermis B. Mesogloea C. Gastrodermis 5. Nervous System 6. Sense organs 7. Locomotion 8. Water circulation 9. Food and feeding 10. Digestive system 11. Respiration and Excretion 12. Reproduction 13. Life Cycle A. Planula larva B. Scyphistoma larva C. Strobilation D. Ephyra Larva E. Metamorphosis 14. Alternation of Generation Table 1 Figures 1-9 V. Polymorphism 1. Introduction 2. Class- Hydrozoa A. Order- Hydroida a. Hydra b. Obelia c. Bougainvillea d. Tubularia e. Hydractinia f. Vellela g. Porpita B. Modifications of polyp C. Modifications of medusa D. Order- Siphonophora Page 3 of 115 a. Physalia b. Diphyes c. Helistemma 3. Class Anthozoa a. Pennatula (Sea Pen) Figures 1-11 VI. Mesenteries 1. Introduction 2. Structure of Metridium (Sea Anemone) explaining Mesenteries A. Primary mesenteries or complete mesenteries B. Secondary mesenteries or incomplete mesenteries 3. Octocorallians 4. Hexacorallians 5. Examples of anthozoans showing different arrangement of mesenteries a. Alcyonium b. Edwardsia c. Gonactinia d. Halcampoides e. Halcampa f. Adamsia g. Haloclava h. Zoanthus i. Epizoanthus j. Cerianthus k. Antipathes (Black coral) l. Peachia m. Metridium 6. Formation of mesenteries 7. Significance and function of mesenteries Figures 1-17 VII. Corals and coral reefs 1. Introduction 2. Class: Anthozoa A. Subclass: Octocorallia or Alcyonacea a. Structure of Octocorallian coral i. Alcyonium (Dead man’s finger or soft coral) ii. Heliopora (blue coral) iii. Tubipora (Organ pipe coral) iv. Corallium (Red coral) v. Gorgonia (Sea fan) Page 4 of 115 B. Subclass: Hexacorallia (Zoantharia) a. Structure and formation of a Hexacorallian coral i. Fungia (Mushroom coral) ii. Madrepora (Horn coral) iii. Astraea (Star coral) iv. Meandrina (Brain coral) v. Astrangia (white coral) vi. Antipathes (Black coral) 3. Class: Hydrozoa i. Millepora (Fire coral or sting coral) 4. Coral reefs A. Development of coral reefs B. Types and structure of Coral reefs a. Fringing reefs b. Barrier reefs c. Atoll C. Theories explaining the formation of coral reefs a. Darwin’s Dana subsidence theory b. Stutchbury’s volcanic crater theory c. Samper Murray solution theory d. Submerged bank theory e. Daly glacial control theory 5. Significance of corals 6. Coral crisis A. Natural factors affecting coral growth a. Temperature b. Depth of the ocean c. Availability of light and presence of algae d. Sea storms B. Man made factors a. Increase in human population and development b. Aesthetic value c. Sea traffic d. Predators C. Coral Bleaching D. Coral Diseases Figures 1-18 VIII. Bibliography IX. Acknowledgements Page 5 of 115 I. Phylum Cnidaria (Coelenterata): General characters 1. Introduction: The phylum coelenterata is a Greek word where- ‘coel’ and ‘enteron’ stand for hollow and intestine respectively. It is also known as Cnidaria because of the presence of a unique organelle called nematocyst or cnidae (formed by special cells called cnidoblasts). The phylum Coelenterata contains more than 10,000 species, including jelly fish, fresh water Hydra, box jellies, sea anemones, soft coral and hard coral forming animals. Cnidarians are Eumetazoans (multicellular) with primarily radial symmetry. They constitute the lowest group of animals among the Eumetazoa. Cnidarians can be distinguished from sponges (see phylum- Porifera) as they have a distinct digestive cavity called coelenteron. They differ from ctenophores (see Phylum- Ctenophora) by having nematocysts, a polypoid stage and reproduce both sexually and asexually. 2. Morphology/ Anatomy: These fascinating animals are basically gelatinous in composition. A majority of them are marine water living except fresh water Hydra and Craspedacusta. Most of the animals are characterized by radial symmetry about an oral- aboral axis. The body presents tissue level organization i.e. cells are organized into tissues which can perform various functions within an organism. The body wall of a cnidarian is diploblastic i.e. composed of only two layers of cells, the ectoderm and the endoderm. A layer of mesogloea generally intervenes between the ectoderm and the endoderm. Although the mesogloea itself is nonliving and gelatinous, it may contain living cells derived from embryonic ectoderm. Amoebocytes in the mesogloea of anthozoans probably play roles in digestion, nutrient transport and storage, wound repair and antibacterial defense. Phylum Cnidaria can be divided into four classes: the Hydrozoa (Hydra, Obelia), the Scyphozoa (jelly fish), Cubomedusae (box jellies), and the Anthozoa (sea anemones, gorgonians, sea pens, corals etc.). Cnidarians may have two basic body forms- polyps and medusae (also called zooids) which may be modified into different forms performing different functions and mutually benefiting each other. Hydrozoans bear polyps or medusa or both, adult of scyphozoans and class Cubomedusae are only medusoid forms, while all anthozoans are polypoid forms. Polyps are generally cylindrical, and are found attached to the main colony or to the substratum while medusae are free living. In polyps the mouth is surrounded by tentacles and faces upwards i.e. away from the substratum. In contrast, medusa is umbrella like, having ex-umbrellar and sub-umbrellar surfaces with the mouth prolonged centrally into a manubrium and facing towards the bottom. Tentacles are present on the periphery of the sub-umbrellar surface. They freely swim in water by rhythmic contractions. Their orientation is inverted in relation to that of the polyps. In all cnidarians, the mouth leads into a spacious cavity called the coelenteron. The coelenteron is enclosed by two–layered body wall (diploblastic condition) consisting of outer ectoderm and inner gastrodersm. Coelenteron is also called the gastrovascular cavity as it acts both as a digestive system and a circulatory system for the circulation of food, oxygen, excretory wastes and many other materials with in the body. It is lined by gastrodermis (also known as the endodermis) which is sometimes infolded to form radial septa or mesenteries. These mesenteries increase the surface area over which digestion may take place, because the primary function of the endoderm is digestion. Cnidarians Page 6 of 115 are mainly carnivorous although some soft corals have been found to feed also on phytoplankton. Colonial forms are fixed, so they cannot move actively from one place to another. They capture their prey with the help of tentacles encircling their mouth. Special stinging cells or cnidoblasts unique to this phylum are present abundantly on the tentacles and at the terminal ends of the mesenteries in anthozoans. Three general types of cnidoblasts are present in different cnidarians. Cnidoblasts having a cnidocil and nematocyst, which is a common type of stinging cells, is present in hydrozoans and scyphozoans. Besides this majority of anthozoans have spirocysts or ptychocysts. Spirocyst, as its name suggests, contains a spirally coiled thread inside. This thread, on discharge, releases sticky material. Spirocysts are present in most of the anthozoans in addition to nematocysts. Similarly, ptychocysts contain a tubule, which is haphazardly arranged inside the capsule, and is released. Ptychocysts are present in the tube dwelling anemones like Cerianthus. These can be discharged with great force for a variety of functions. Around 30 different types of cnidoblasts are reported to be present in different cnidarians. Many cnidarians have symbiotic photosynthetic algae i.e. zoochlorallae (in Hydra) and zooxanthellae (in marine cnidarians) within their gastrodermal cells or in their ectodermal cells. They live symbiotically with these algae and obtain additional nutrients from them. They benefit algae in turn by providing CO2 and metabolic products for photosynthesis. The nervous system is very primitive and lacks a central nervous system. In contrast it consists of a network of nerve cells and nerve processes which generally synapse on one another repeatedly and terminate at a neuromuscular junction. This type of a nerve network helps in immediate spread of excitation over the entire body of the animal when sensory cells are stimulated. As a result, the animal can change its orientation or react according to the situation. Muscle layers are derived from epitheliomuscular cells and endotheliomuscular cells that possess elongated,