Biology II - Chapter 13: Radiate Animals PDF

BIOLOGY II CHAPTER 13 RADIATE ANIMALS  RADIATE ANIMALS STUDY OBJECTIVES RADIATE The student should be able to understand the following concepts: ANIMALS 1. cnidarian characteristics, Phyla Cnidaria a...

BIOLOGY II CHAPTER 13 RADIATE ANIMALS  RADIATE ANIMALS STUDY OBJECTIVES RADIATE The student should be able to understand the following concepts: ANIMALS 1. cnidarian characteristics, Phyla Cnidaria and 2. characteristics of all classes, Ctenophora. 3. polymorphism, Radiate animals: 4. type of cnidarian cells, 5. life cycle of Obelia, primary radial or 6. life cycle of Aurelia, biradial symmetry. 7. coral reef types, 8. Ctenophora characteristics, 9. phylogeny of Cnidaria PHYLUM CNIDARIA (COELENTERATA) CHARACTERISTICS Radial or biradial symmetry around longitudinal axis with oral and aboral ends. Polymorphism (polype + medusa). Exoskeleton / endoskeleton of chitinous, calcareous or protein components. Body of 2 germ layers – diploblastic. Outer epidermis & inner gastrodermis. Gastrovascular cavity with single opening (mouth). Stinging cells (nematocysts), colloblasts and ciliary comb plates. Nerve net with nerve cells (protoneurons).  Muscular system: epitheliomuscular cells, longitudinal fibers & circular fibers. Extracellular digestion in gastrovascular cavity. Sense organs: statocyst & ocelli. Asexual reproduction: budding. Sexual reproduction: gametes, planula larvae. No excretory system – diffusion. No respiratory system – diffusion. No coelomic cavity. DIMORPHISM & POLYMORPHISM IN CNIDARIANS Dimorphism - a species with two distinct forms according to color, sex, size, organ structure, etc. Polymorphism - a species with more than one structural type of individual. All cnidarian forms fit into one of two morphological types (dimorphism):  polyp (hydroïd form), adapted to sedentary or sessile life,  medusa (jellyfish form), adapted for floating or free-swimming existence. Cnidarian Body Forms: Polyp Asexual Sessile Cylindrical, with mouth and tentacles opening upward Medusa Dioecious (separate sexes) Free-swimming Bell-shaped, with mouth and tentacles facing downward Polymorphism = diversity in form and function of individuals in a colony when more than one body forms are present at the same stage in the life cycle. Example of polymorphism: Obelia: gastrozooïds perform all functions except reproduction. gonozooïds are solely reproductive, forming medusa for dispersal. Physalia: Each colony includes at least 4 types of polyps: (a) pneumatophore or float, gas is secreted to render buoyancy; (b) feeding polyps = gastrozooïds; (c) defensive or fighting polyps with nematocysts = dactylozooïds; (d) reproductive polyps = gonozooïds. CLASSIFICATION Phylum Cnidaria Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa CLASS HYDROZOA BODY PLAN Cylindrical tube with aboral end forming stalk, ending in basal or pedal disc for attachment. Mouth located on hypostome, surrounded by 6 - 10 hollow tentacles. Mouth opens into gastrovascular cavity. BODY WALL Body wall consists of: outer epidermis (ectodermal), inner gastrodermis (endodermal), with thin mesoglea. EPIDERMAL CELL TYPES a) Epitheliomuscular cells: for covering & muscular contraction (myofibrils). b) Gland cells: secrete adhesive substance for attachment, and gas bubble for floating. c) Interstitial cells: undifferentiated cells, give rise to cnidoblasts, sex cells, buds, nerve cells etc. d) Cnidocytes: contain nematocysts. Nematocysts - the stinging organelles Self study (Hickman et al., 2017, p. 264-265).  Nematocyst is a capsule, containing a coiled tubular filament, or "thread". Anterior end of capsule covered by operculum. Undischarged thread bears tiny barbs / spines. Nematocyst enclosed in cell, the cnidocyte or cnidoblast. Cell has a projecting hair-like "trigger" (cnidocil). Cnidocytes are most abundant on the tentacles. Self study: mechanism of nematocyst discharge. Three types of nematocysts:  volvents wrap around an object;  penetrants actually penetrate tissues of the prey;  glutinants discharge adhesive substances. e. Sensory cells: touch, temperature & chemicals for detecting among epitheliomuscular cells. Free ends flagellated, other ends branch into fibrils attached to nerve net. f. Nerve cells (multipolar) at base of epidermis connect with sensory cells, muscle fiber, etc. and form nerve plexus & nerve net. (ii) GASTRODERMAL CELL TYPES Gastrodermis lines the gastrovascular cavity. a) Nutritive-muscular cells: Cells contain myofibrils, forming circular muscle fibers. Cells filled with food vacuoles. Flagella ensure continual movement of food and fluids in digestive cavity. Water serves as hydrostatic skeleton. b) Interstitial cells: transform in other cells when needed. c) Gland cells secrete digestive enzymes and mucus. (iii) Mesoglea Support body and gives rigidity to body. Consists of jelly /mostly water - with mesenchymal cells & connective tissue fibers. LOCOMOTION Hydra glides on basal disc, aided by mucous secretions. It can bend over, using tentacles to attach to substratum and move basal disc. NUTRITION & DIGESTION Carnivorous, tentacular feeders, ciliary feeders: – trap organisms in mucus on their bodies or tentacles; convey them to mouth by ciliary action. Glutathione - activator to open mouth. Digestion:  extracellular in gastrovascular cavity;  intracellular in nutritive-muscular cells. Anal opening absent; indigestible parts regurgitated through mouth. NERVE NET Diffuse network of neurons under epithelial layers (nerve net). Neurons - bipolar (with 2 processes) or multipolar (with many processes). Sensory cells feed into nerve plexus, connecting with muscle fibers. Nerve cells both at base of epidermis and gastrodermis. REPRODUCTION Asexual reproduction:  bud formation &  regeneration powers.  Sexual reproduction:  dioecious (have separate sexes). Sex cells originate from interstitial cells in epithelium. Gonads appear in autumn, stimulated by lower temperatures & reduced aeration of stagnant waters. Fertilization is external in surrounding water. Zygote undergoes holoblastic cleavage to form hollow blastula. Inner part of blastula form gastrodermis. Cyst (to survive winter) forms around embryo, then breaks loose. Young hydras hatch out in spring, when weather is favorable. RESPIRATION & EXCRETION No respiratory and excretory organs in cnidarians. Gas exchange and elimination of liquid wastes by diffusion across cell membranes. HYDROID COLONIES Physalia: FLOATING COLONIES Examples: Physalia (Portuguese man-of-war)  Float - pneumatophore, contains air sac filled with gas. Types of polyp individuals:  feeding polyps (gastrozooïds),  reproductive polyps (gonophores),  fighting polyps (dactylozooïds). Obelia: Obelia: hydroïds with a medusa stage. Hydroïd with base, stalk and 1 or more zooids. Base attached to substratum with root-like stolon, or hydrorhiza, gives rise to 1 or more stalks, e.g. hydrocauli. Living cellular part of hydrocaulus = coenosarc. Protective covering of hydrocaulus = perisarc. Attached to hydrocaulus are individual polyps, or zooïds. Some zooïds are feeding polyps, called hydranths or gastrozooïds, all with a terminal mouth and circlet of tentacles. Hydranths capture & ingest prey, digested in gvc (intracellular digestion).  Asexual reproduction: budding. Sexual reproduction:  Obelia medusae bud from a reproductive polyp - gonangium.  Medusae reproduce sexually.  Young medusae leave colony as free-swimming individuals that mature and adult male & female medusa produce gametes (eggs and sperm).  Gonads suspended beneath each radial canal.  Fertilization may be external, with both eggs and sperm shed into water, or fertilization and development of egg may occur in medusa.  Embryonation of zygote form ciliated planula larva, swims around, settles on substratum, develop into polyp giving rise through asexual buds to hydroid colony. Ciliated planula larvae: Medusa: Margin of bell of medusae projects inward to form velum, used to swim. Tentacles attached to bell margin, with nematocysts. Mouth opens at end of manubrium, leading to stomach and 4 radial canals that connect with a ring canal around margin. GVC is continuous from mouth to tentacles and lined with gastrodermis. Muscular system is best developed around bell margin and velum. Nerve net concentrated into two nerve rings at base of velum. Bell margin supplied with sensory cells. Two types of sense organs: statocysts (small organs of equilibrium), and ocelli (light-sensitive organs). CLASS SCYPHOZOA Called large jellyfishes, polyp stage reduced or absent.  Mouth is centered on sub-umbrella side. Manubrium drawn out into 4 oral arms - capture food & ingestion. Feed on small organisms, form protozoans to fishes. 4 gastric pouches contain nematocysts on gastric filaments connect to stomach and complex system of radial canals that branch from pouches to ring canal and complete gastrovascular cavity, through which nutrients circulate. Margin of bell bears sense organ (rhopalium) and pair of lob-like projections called lappets. Aurelia has 8 sense organs (notches). Scyphomedusae have no velum.  Locomotion of scyphomedusae:  powerful circular muscles around margin of bell.  muscle fibers in medusae are striated. Nerve net synaptic. Sense organs:  concentrated in rhopalia;  bearing a statocyst (balance),  2 sensory pits (concentrating sensory cells);  ocellus (eyespot) for photoreception.  Sexual reproduction:  Sexes are separate.  Fertilization internal.  Zygotes develop into ciliated planula larva, it swims, settles and attaches to sea bottom, lose cilia, and it bears a polypoïd larva (scyphistoma), with basal disk, mouth and tentacles.  Scyphistoma produces more scyphistomas by budding.  In winter and summer it buds off to form a strobila by a process called strobilation, forming a series of minute buds, called ephyrae, that break loose, swim and grow into mature, sexual medusae (jellyfish). CLASS CUBOZOA  Medusa is predominant form, polyp inconspicuous. Tentacles or group of tentacles found at each corner of square umbrella margin. Base of tentacle form flat, tough blade, called pedalium. Rhopalia present. Sub-umbrella edges turn inward, form velarium (increase swimming efficiency). CLASS ANTHOZOA All are polypoïd with no medusa stages. Three subclasses:  Hexacorallia (Zoantharia) (sea anemones & hard corals):  tentacles & mesenteries arranged on plan of 6 or multiples of 6 (hexamerous) and tentacles are un-branched.  Ceriantipatharia (tube anemones & thorny corals):  tentacles and mesenteries arranged on plan of 6 or multiples of 6 (hexamerous) and tentacles un-branched.  Octocorallia (Alcyonaria) (soft and horny corals - sea pens, sea pansies and sea fans):  octamerous and have 8 branched tentacles.  Large gastrovascular cavity, partitioned by longitudinal mesenteries (septa), into compartments. Walls and mesenteries have both circular & longitudinal muscles. Mesoglea is mesenchyme containing ameboid cells. Sea Anemones  Cylindrical, crown of hollow tentacles around the mouth of flat oral disc.  Ciliated groove (siphonoglyph) leads into pharynx. Siphonoglyph creates water current (bring in oxygen-rich water) and cilia on rest of pharynx carry water and waste produces out.  GVC divided by 6 pairs of primary (complete) septa (hexamerous), subdivided by smaller (incomplete) septa.  Free edges of septa form cord (septal filament), bearing nematocysts.  Base of septa has acontia threads (with nematocysts).  Asexual reproduction: budding, pedal laceration (regeneration powers), longitudinal fission & transverse fission.  Sexual reproduction: sexes separate, some hermaphroditic, monoecious species = protandrous (sperm produced first, then eggs), gonads on margin of septa, fertilization internal or external, zygote develops into ciliated larva.  Hexacorallia (Zoantharia) Corals True or stony corals live in calcareous cups that form exoskeleton, are secreted by epidermis, with un-branched tentacles. GVC subdivided by septa in multiples of six (hexamerous). No siphonoglyph. Epidermis at base of column secretes limy skeleton cup. Most corals live in colonies. Asexual reproduction - budding of new polyps. Sexually reproduction - producing ciliated larvae, starting new colonies elsewhere.  Tube anemones & thorny corals (Ceriantipatharia) Simple un-branched tentacles. Hexamerous, with coupled but unpaired septa (mesenteries). Tubes constructed of secreted mucus & threads of organelles. Octocorallia (Alcyonaria) corals Octomerous symmetry. Have 8 pinnate tentacles and 8 unpaired complete septa. GVC of polyps communicate through system of gastrodermal tubes, called solenia, that run through mass of mesoglea (coenenchyme) and is connected by a system of canals. Skeleton secreted in coenenchyme, forming endoskeleton.  Coral reefs Types of reefs:  fringing reef: Is closer to land, with either no lagoon or a narrow lagoon between it and the shore. Fringing reefs are relatively young. They can develop in shallow waters along the coast of tropical islands or continents. The corals grow upwards to sea level or just below and outwards towards the open ocean. Fringing reefs are generally narrow platforms a short distance from shore and don't contain a substantial lagoon.  fringing reef: Is closer to land, with either no lagoon or a narrow lagoon between it and the shore.  Runs parallel to the shore and has a wider and deeper  barrier reef: lagoon than a fringing reef. A coral reef growing parallel to the coastline and separated from it by a lagoon is called a barrier reef. The lagoon may develop between the fringing reef and the land. As the reef continues to grow further and further offshore it eventually reaches the edge of the continental shelf. Barrier reefs can also originate offshore if the depth of the seabed out there is shallow enough to allow corals to grow. The most famous barrier reef is the Great Barrier Reef in Australia. It stretches over 2300 km and covers over 200'000 km2. It lies between 24 and 240 km from the main continent.  barrier reef: Runs parallel to the shore and has a wider and deeper lagoon than a fringing reef.  atoll: Reefs that encircle a lagoon, but not an island. Atolls are rings of reef, often encircling an island (sand and coral rubble). They typically have a shallow, sandy, sheltered lagoon in the middle. Access to the open sea beyond is through a number of channels. These provide fresh and colder water for the lagoons. Corals atolls are on the top of submarine mountains. These mountains are remnants of volcanoes. Once there were fringing reefs around the volcano. As it slowly submerged the corals continued to grow up to the surface of the water. What remained after the volcano became invisible is a ring of coral reefs surrounded by a deep ocean.  atoll: Reefs that encircle a lagoon, but not an island.  patch or bank reefs: Occur some distance back from the steep, seaward slope in lagoons of barrier reefs or atolls. Platform reefs usually lie in sheltered seas and quite far offshore. They are flat-topped with small and very shallow lagoons.  Reef terms: Draw a typical reef:  reef front or fore reef slope: Side of reef facing the sea. Reef front is parallel to the shore and perpendicular to the direction of that waves travel.  reef crest: Top of reef front. Bear the greatest force of waves and must absorb great energy during storms.  reef flat: Slopes down into the lagoon.  Hydrozoa:  mesoglea is acellular.  cnidocytes limited to epidermis.  gastrovascular cavity is simple (non-septate). Scyphozoa & Anthozoa: PHYLOGENY  mesoglea contains cells.  gastrodermal cnidocytes.  gastrodermal gonads.  a septate gastrovascular cavity. Scyphozoa - with medusa stage. Anthozoa - medusa became suppressed. PHYLUM CTENOPHORA CHARACTERISTICS Biradial symmetry. Rows of comb plates for swimming. Triploblastic. Nematocysts absent, adhesive cells (colloblasts) for food capture. Digestive system: mouth, pharynx, stomach, a series of canals & anal pore. Nervous system: sub-epidermal nerve plexus around mouth & under comb plate rows. Sensory system: an aboral sense organ (statocyst). No polymorphism. Reproduction: monoecious; gonads (endodermal) on walls of digestive canals, cydippid larva. Cydippid larva: COMPARISON WITH CNIDARIA Ctenophores resemble cnidarians in following ways: Radial symmetry. Aboral-oral axis around which parts are arranged. Well developed gelatinous ectomesoderm (collenchyme). No coelomic cavity. Diffuse nerve plexus. Lack of organ systems. Ctenophores differ from cnidarians in following ways: No nematocysts. Have colloblasts (adhesive cells). Presence of comb plates for swimming. Development of muscle cells from mesenchyme. Mosaic, or determinate type of development. Presence of pharynx. No polymorphism. Never colonial. Presence of anal openings. CLASSIFICATION a) CLASS TENTACULATA With tentacles, with or without sheaths into which they retract, compressed in tentacular plane. Examples: Pleurobranchia, Mnemiopsis, Cestum. Pleurobranchia CLASS NUDA Without tentacles. Flattened in tentacular plane. Wide mouth and pharynx; gastrovascular canals much branched. Example: Beroe. Beroe PHYLOGENY Origin of cnidarians and ctenophores obscure. Might arise from a radially symmetrical planula-like ancestor. A planula larva in which an invagination formed to become the gastrovascular cavity would correspond roughly to a cnidarian with an ectoderm and an endoderm. The trachyline medusae are often believed to resemble the ancestral cnidarian because of their direct development from the planula larvae to the medusa. The trachyline-like ancestors would have given rise to other cnidarian lines after the evolution of the polyp stage and alternation of sexual (medusa) and asexual (polyp) generations. Subsequently, the medusa was lost in the anthozoan. SUMMARY Study Hickman et al., 2017, p. 287. REVIEW QUESTIONS Students should be able to answer Questions 1 - 23 (Hickman et al., 2017, p. 287 - 288). 1. Characteristics of phylum Compare the characteristics of the phylum with Cnidaria. any other taxa 2. Basic cnidarian Define the following terms: nematocyst, terminology. operculum, cnidocyte, cnidocil, hypostome, gastrovascular cavity, perisarc, gonangium, velum, manubrium, statocyst, ocelli, pneumatophore, gastrozooïds, dactylozooïds, gonophore, lappets, rhopalium, scyphistoma, strobila, ephyra, velarium, pedalium, acontia, protandrous, solenia, and atoll. 3. Polymorphism in Cnidaria. Discuss polymorphism in Cnidaria. Distinguish with sketches between the polyp and medusa forms. 4. Types, morphology and Name and distinguish between the different function of nematocysts. types of nematocysts. Make a labelled sketch of a nematocyst. Explain the mechanism of nematocyst discharge. 5. Cell types in epidermis Name and describe the cell types in the and gastrodermis. epidermis. Name and describe the cell types in the gastrodermis. 6. Classes of Cnidaria. Name and give the differences between the cnidarian classes. 7. Life cycle of Obelia. Describe and illustrate the life cycle of Obelia. 8. Polymorphism in Physalia. Describe polymorphism in Physalia. 9. Life cycle of Aurelia. Describe and illustrate the life cycle of Aurelia. 10. Subclasses of Anthozoa. In table form name the subclasses of Anthozoa and indicate the differences between them. 11. Reef types. Name and discuss the different types of reefs. 12. Phylogenetic development Discuss the phylogenetic development of of Cnidaria. Cnidaria. Draw a cladogram to indicate the hypothetical relationship between the cnidarian classes. 13. Characteristics of Phylum Compare the characteristics of the phylum Ctenophora. Ctenophora with any other taxa. 14. Basic Ctenophora Define the following terms: comb plates, comb terminology. rows and colloblasts. 15. Cnidaria vs. Ctenophora. Give in table form the differences and similarities between Cnidaria and Ctenophora. THE END