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CHAPTER 13 Radiate Animals 13-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Over 9,000 species in the phylum Cnidaria Equipped with specialized cells: cnidocytes Fossil specimens dated to over 700 million years ago Extant species 13-3 Contain a specialized stinging organelle, the nematocyst Most common in shallow marine environments Some freshwater None are terrestrial Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Some ctenophores, molluscs and flatworms eat hydroids and use the stinging nematocysts for their own defense Four classes of Cnidaria Hydrozoa Scyphozoa Cubozoa Anthozoa A fifth class, Staurozoa, has been proposed 13-4 No medusae in life cycle but polyp topped by medusa-like region Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Characteristics of Phylum Cnidaria All are aquatic and mostly marine Symmetry Radial or biradial Two body types Free-swimming medusae Sessile polyps Diploblastic Epidermis and gastrodermis Mesoglea: extracellar matrix that lies between ectodermis and gastrodermis 13-5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Incomplete gut: gastrovascular cavity Tentacles usually encircle mouth or oral region Muscular contractions via epitheliomuscular cells 13-6 Extracellular digestion in gastrovascular cavity Intracellular digestion in gastrodermal cells Outer layer of longitudinal fibers and an inner layer of circular fibers Sense organs for balance (statocysts) and photosensitivity (ocelli) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Nerve net Asexual reproduction 13-7 Budding in polyps Some colonies exhibit polymorphism Sexual reproduction Symmetrical and asymmetrical synapses Diffuse conduction Two nerve rings in hydrozoan medusae By gametes in all medusae and some polyps Monoecious or Dioecious; Holoblastic indeterminate cleavage No excretory or respiratory system Acoelomate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Form and Function Cnidaria have two basic body plans: polyp and medusa Polyp Hydroid form Adaptation to a sedentary life Tubular body with the mouth directed upward and surrounded by tentacles Mouth leads into a blind gastrovascular cavity Aboral end attached to substratum by pedal disc Reproduce asexually by budding, fission, or pedal laceration 13-10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria In colonial forms In class Hydrozoa 13-11 Polyps may be specialized for feeding, reproduction, or defense feeding polyps (hydranths) distinguished from reproductive polyps (gonangia) by absence of tentacles in gonangia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Medusa Bell or umbrella-shaped Usually free-swimming Mouth directed downward Tentacles may extend down from rim of umbrella Medusae equipped with statocysts and ocelli Integration of sensory information into motor response Velum differentiates hydromedusae from scyphomedusae 13-12 Function of a nerve ring located at base of the bell Shelf-like fold of tissue from the bottom of the bell that extends into the bell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Life Cycles Polyps and medusae play different roles in the cnidarian life cycle Typically, zygote develops into a motile planula larva Planula settles, and metamorphoses into a polyp 13-14 Produce other polyps asexually Polyps eventually produce a free-swimming medusa by asexual reproduction Budding or strobilation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Medusae Dioecious Reproduce sexually True jellyfish (class Scyphozoa) Most colonial hydroids 13-15 Medusa is large and conspicuous Polyps typically very small Feature a polyp stage and a pelagic medusa stage Some hydrozoans (Physalia) form floating colonies In Hydra, only stage is a small freshwater polyp Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Body Wall Cnidarian body 13-16 Outer epidermis Inner gastrodermis Layers separated by mesoglea Mesoglea Gelatinous (at least 95% water) Continuous in polyps, extending through body and tentacles Supports body Thick in anthozoans, and scyphozoan medusae Thinner in hydromedusae Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria In Hydra Epidermis contains epitheliomuscular, interstitial, gland, sensory, and nerve cells Cnidarian bodies extend contract, bend, and pulse No mesodermally derived muscle cells Have epitheliomuscular cells 13-17 Form most of epidermis Cover organism and function in muscle contraction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-18 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Cnidocytes Many cnidarians are effective predators Possible due to presence of a unique cell type, the cnidocyte Cnidoctyes 13-19 Located in invaginations of ectodermal cells and some gastrodermal cells Produces one of over 20 types of cnidae One type of cnida is the nematocyst Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Nematocyts 13-20 Tiny capsules made of chitin-like material and containing a coiled filament End of capsule is covered Filament may have tiny barbs or spines Except in Anthozoa, cnidocytes equipped with trigger-like cnidocil (modified cilium) Tactile stimulation cause nematocyst to discharge After Cnidae discharge, cnidocyte is absorbed and another develops Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Mechanism of Nematocyst Discharge 13-21 When stimulated, water to rush into the capsule The operculum opens and rapidly launches the filament Barbs inject poison into prey Only a few jellyfish and the Portuguese manof-war can seriously harm humans Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-22 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-23 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Feeding and Digestion Polyps 13-24 Typically carnivorous Catch prey with tentacles and pass them to the gastrovascular cavity Gland cells discharge enzymes to begin extracellular digestion Intracellular digestion continues in the gastrodermis Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Polyps in colonial hydrozoans Feeding and digestion in hydromedusae Similar to that seen in the polyps Schyphomedusae Pass food into a common gastrovascular cavity Equipped with extended mouth edge (manubrium) Used in capturing and ingesting prey Anthozoan polyps Carnivorous Expand and stretch tentacles in search of prey Some small anthozoans 13-25 Feed on minute forms captured by ciliary currents Corals supplement their nutrition with carbon collected from algal symbionts Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Nerve Net Nerve net of cnidarians one of the best examples of diffuse nervous system Two nerve nets, one at the base of epidermis and one at the base of gastrodermis, interconnect Nerve action potentials transmitted across synapses by neurotransmitters Unlike higher animals, 13-26 Nerve nets have neurotransmitters on both sides of the synapses Allowing transmission in either direction No myelin sheath on axons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Nerve cells synapse with both slender sensory cells and epitheliomuscular Association often called a neuromuscular system The nerve net pattern is also found in annelid and human (nerve plexus) digestive systems 13-27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Class Hydrozoa Most marine and colonial with both polyp and medusa forms Hydra is not typical Colonial Obelia is more exemplary 13-28 Typical hydroid has a base, a stalk, and one or more terminal zooids (individual polyp animals) Base is a rootlike stolon, or hydrorhiza Gives rise to stalks called hydrocauli Living part of the hydrocaulus is a tubular coenosarc Hydrocaulus covered by a non-living chitinous sheath, the perisarc Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Individual zooids are attached to the hydrocaulus Hydranths (gastrozooids) are feeding polyps with circle of tentacles surrounding mouth Hydranths may be thecate or athecate Colonial hydroids bud off new individuals Individuals may be new hydranths or medusae buds In Obelia, the medusae buds are formed by a reproductive polyp called a gonangium 13-29 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-31 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Hydroid medusae Usually smaller than schyphozoan medusae Margin of the bell projects inward as a shelf-like velum Mouth opens at the end of a suspended manubrium Mouth connects to a stomach and four radial canals Radial canals connect to a ring canal that runs around the margin of the bell and connects with the hollow tentacles Bell margin has many sensory cells Typically also bears statocysts, specialized sense organs that function in equilibrium, and light-sensitive ocelli 13-32 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-33 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-34 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-35 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Freshwater Medusae Craspedacusta May have evolved from marine ancestors Found in Europe, the United States, and parts of Canada Craspedacusta medusae May reach a diameter of 20 mm Polyps are tiny (2 mm) Polyp employs three methods of asexual reproduction 13-36 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-37 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Hydra Found on the underside of aquatic leaves and lily pads in clean fresh water Worldwide distribution (16 species in North America) Body is a cylindrical tube Aboral end has a basal or pedal disc for attachment The mouth (oral end) on a conical elevation, the hypostome Ring of 6–10 hollow tentacles encircles mouth 13-38 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria The mouth opens to a gastrovascular cavity Buds may project from the side, each develop a mouth and tentacles Hydras feed on a variety of small crustaceans, insect larvae, and worms Mouth is located on a raised hypostome, and opens into the gastrovascular cavity Food organisms brush against the tentacles are captured by nematocysts 13-39 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Organism moved by tentacles, and engulfed by the mouth Opening of mouth activated by glutathione Inside gastrovascular cavity, gland cells discharge enzymes Myofibrils in nutritive-muscular cells run at right angles to the body axis, forms a weak circular muscle layer Water enters the gastrovascular cavity due to beating cilia 13-40 Water serves as a hydrostatic skeleton Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-41 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Epitheliomuscular cells Form most of epidermis and cause muscular contraction Undifferentiated interstitial cells Develop into cnidoblasts, sex cells, buds, or nerve cells, but not epitheliomuscular cells. Gland cells On the adhesive disc secrete an adhesive and sometimes a gas bubble for floating 13-42 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Hydras have three types of cnidae Penetrants penetrate prey and inject poison Volvents recoil and entangle prey Glutinants secrete an adhesive for locomotion and attachment Sensory cells among epidermal cells bear a flagellum for chemical and tactile stimuli and synapse with nerve cells Epidermal nerve cells are generally multipolar with both one-way and two-way synapses. 13-43 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria 13-44 Hydras reproduce sexually and asexually Asexual reproduction Budding Most hydra are dioecious Temporary gonads appear in autumn, stimulated by lower temperatures or stagnation Eggs and sperm shed externally Zygotes undergo holoblastic cleavage to form hollow blastula Cyst forms around embryo Encysted form endures the winter, then young hydras hatch in the spring Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-45 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Other Hydrozoans 13-46 Orders Siphonophora and Chondrophora form polymorphic swimming or floating colonies Contain several types of polyp individuals Dactylozooids are Fishing tentacles that sting prey and lift them to feeding polyps Gonophores Sacs containing ovaries or testes. In Physalia, the float, pneumatophore Thought to have expanded from the original larval polyp Other hydrozoans secrete calcareous skeletons resembling true corals and are the hydrocorals Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-47 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-48 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Class Scyphozoa Most of the larger jellyfishes belong to this class Nearly all float in open sea Bells vary in shape and size One order is sessile, attached to seaweeds by a stalk Composed mostly of mesoglea Mesoglea contains ameboid cells and fibers Lack shelf-like velum found in hydrozoan medusae Margin of the umbrella has indentations, each bearing a pair of lappets 13-49 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-50 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-51 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Between lappets is a equilibrium sense organ Mouth located beneath the umbrella Manubrium forms four oral arms Rhopalium Contains a hollow statocyst Capture and ingest prey Tentacles, manubrium, and often entire body may have nematocysts 13-52 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria The nervous system consists of a nerve net Sexes are separate Fertilization is internal in the gastric pouch of the female Zygote develops into a ciliated planula larva Attaches and develops into a scyphistoma Scyphistoma undergoes strobilation Form buds called ephyrae that break loose to form jellyfish medusae 13-53 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-54 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Class Staurozoa Commonly called stauromedusans No medusa stage Solitary polyp body that is stalked Uses adhesive disk to attach to seaweeds, and objects on sea bottom Polyp top resembles a medusa with eight extensions (“arms”) ending in tentacle clusters surrounding mouth Reproduce sexually 13-55 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-56 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Class Cubozoa Medusa form is dominant Polyp is inconspicuous or unknown Umbrella is square One or more tentacles extend from each corner At base of each tentacle is a flat blade called a pedalium Umbrella edge turns inward to form a velarium 13-57 Increases swimming efficiency Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Strong swimmers Feed mostly on fish in nearshore areas Polyp stage is tiny New polyps bud laterally Do not produce ephyrae but directly change into medusae The sea wasp 13-58 Potentially fatal cubomedusan from Australia Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-59 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Class Anthozoa Lack a medusa stage All marine, in both deep and shallow water, and vary in size Three subclasses: Zoantharia, Cerianthpatharia, and Alcyonaria Zoantharia and Cerianthpatharia are hexamerous; Alcyonaria are octomerous Gastrovascular Cavity 13-60 Large and partitioned by septa or mesenteries, inward extensions of body wall Septa may be coupled or paired Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-61 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-62 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-63 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria The mesoglea is mesenchyme containing ameboid cells No special organs for respiration or excretion Sea Anemones Polyps larger and heavier than hydrozoan polyps Attach to shells, rocks, timber, etc. by pedal discs 13-64 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Some burrow in mud or sand Crown of tentacles surrounds the flat oral disc Slit-shaped mouth leads into a pharynx Siphonoglyph (ciliated groove) creates a water current directed into the pharynx Transports oxygen, removes wastes, and maintains fluid pressure for a hydrostatic skeleton 13-65 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Gastrovascular cavity divided into six pairs of primary septa or mesenteries Smaller incomplete septa subdivide the large chambers increasing surface area Free edge of each incomplete septum Forms a septal filament with nematocysts and gland cells for digestion Acontia threads at lower ends of septal filaments 13-66 Equipped with nematocysts May protrude through mouth to help secure prey Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria When in danger, water rapidly expelled through pores as the anemone contracts to a small size Feeding behavior under chemical control Asparagine activates feeding causing tentacles to bend toward the mouth Reduced glutathione induces swallowing Longitudinal muscles of the epidermis Only occur in the tentacles and oral disc Longitudinal muscles of the column Gastrodermal and located in the septa Most anemones can glide slowly on pedal discs 13-67 Some can swim with limited ability Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-68 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria When in danger, they contract and withdraw tentacles and oral discs Most harbor symbiotic algae Some have a mutualistic relationship with hermit crabs 13-69 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-70 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Reproduction Some dioecious, some monoecious Monoecious species are protandrous Produce sperm first and eggs later Gonads on margins of septa Fertilization is external or in gastrovascular cavity Zygote becomes a ciliated larva Pedal laceration, small pieces of pedal disc break off and regenerate a small anemone Longitudinal and transverse fission, and budding occur 13-71 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Hexacorallian Corals Members of the order Scleractinia Also called true or stony corals Described as miniature sea anemones that live in calcareous cups they have secreted Gastrovascular cavity is hexamerous No siphonoglyph No pedal disc Secrete a limey skeletal cup with sclerosepta projecting up into the polyp Sheet of living tissue forms over the coral surface Connects all gastrovascular cavities 13-72 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-73 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-74 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-75 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Tube Anemones and Thorny Corals Members of subclass Ceriantipatharia Have coupled but unpaired septa. Tube anemones Thorny or black corals Solitary and live in soft sediments Colonial and attach to firm substrata Both groups have few species and live in warmer seas 13-76 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-77 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-78 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Alcyonarian (Octocorallian) Corals Octomerous symmetry, All are colonial Eight pinnate tentacles Eight unpaired complete septa Gastrovascular cavities communicate through tubes called solenia Solenia pass through an extensive mesoglea Show great variation in form of colony 13-79 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-80 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-81 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-82 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Coral Reefs Great diversity of organisms Rivaled only by tropical rainforests Plants and animals are limited to top layer Above the calcium carbonate deposits Hermatypic corals and coralline algae form most coral reefs Require warmth, light, and salinity of undiluted sea water Limited to shallow waters between 30 degrees north and 30 degrees south latitude Photosynthetic zooxanthellae live in their tissues Provide food for corals and recycle phosphorus and nitrogenous wastes 13-83 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Types of Reefs Fringing Reef Barrier Reef Near the land with no lagoon or a very narrow lagoon Parallel to shore with a wide and deep lagoon Atolls Encircle a lagoon but not an island Have a steep bank on the seaward slope Patch or Bank Reefs 13-84 Some distance back from any steep slopes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Reef front or fore reef slope Reef crest Side facing the sea Shallow water or emergent at top of the reef front Wave action breaks pieces off Reef flat toward the shore receives debris and coralline sand Support a diversity of corals and fish Few nutrients enter or leave the system 13-85 Little is lost efficiency in recycling among interacting organisms Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-86 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Nutrients from fertilizer and sewage threaten coral reefs with excessive algal growth Persian Gulf reefs have withstood surprising amounts of oil pollution Coral reefs in many areas are threatened by factors mostly of human origin Higher atmospheric concentrations of carbon dioxide (from burning hydrocarbon fuels) tends to acidify ocean water, which makes precipitation of CaCO3 by corals more difficult metabolically 13-87 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Cnidaria Classification of Cnidaria Class Hydrozoa Class Scyphozoa Class Staurozoa Class Cubozoa Class Anthozoa Subclass Hexacorallia (Zoantharia) Subclass Ceriantipatharia Subclass Octocorrallia (Alcyonaria) 13-88 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Phylum composed of about 150 species All marine, most prefer warm waters Ctenophores Eight rows of comb-like plates used for locomotion Nearly all free-swimming, few creep or are sessile Body structure (ellipsoid or spherical shape) 13-89 Biradial symmetry due to presence of two tentacles. Oral-aboral axis, no head Translucent body with a gelatinous layer that contains muscle fibers (fibers are radial, meridional, and latitudinal bands) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Complete gut Acoelomate Feeding Habits 13-90 Trailing tentacles capture planktonic organisms by means of epidermal glue cells called colloblasts Short tentacles collect food on the ciliated body surface Ctenophores without tentacles feed on other gelatinous animals Structuring classes within the Ctenophores still being developed Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Representative Type: Pleurobrachia Transparent and 1.5–2 cm in diameter Oral pole bears the mouth opening Aboral pole equipped with statocyst Eight equally-spaced bands called comb rows extend from aboral to oral pole Each band made of transverse plates of long fused cilia called comb plates Beat in each row begins at the aboral end and moves along combs to the oral end All eight rows beat in unison Drives the animal forward mouth-first 13-91 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-92 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-93 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Tentacles Two tentacles are long, solid and extensible Retract into a pair of tentacle sheaths Surface bears colloblasts or glue cells that secrete sticky material to hold animals Body wall resembles cnidarians with a gelatinous collenchyme in the interior Muscle cells are distinct and not part of the epitheliomuscular cells 13-94 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Digestion and Feeding Gastrovascular system comprises a mouth, pharynx, stomach and canals that run to the comb plates, tentacular sheaths, and elsewhere Two blind canals terminate near mouth Aboral canal divides into two small anal canals that expel wastes Digestion both extracellular and intracellular 13-95 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Respiration and excretion occur through body surface Nervous and Sensory Systems Resembles cnidarians Subepidermal plexus concentrated under each comb plate Statocyst is a bell-like chamber 13-96 Tufts of cilia sense changes in pressure from statolith as animal changes position Epidermis bears sensory cells sensitive to chemical and other stimuli Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora When ctenophore contacts an unfavorable stimulus Cilia reverse their beat Moves organisms backward Comb plates are sensitive to touch 13-97 Withdraw into the animal when touched Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Reproduction and Development 13-98 Monoecious Fertilized eggs discharged through epidermis into water Cleavage is determinate Free-swimming cydippid larva somewhat resembles adult Some consider cellular nature of mesoglea to constitute a mesoderm If mesoderm is derived from endoderm, both ctenophores and cnidarians are diploblastic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-99 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylum Ctenophora Other Ctenophores Beroe Venus’ girdle Band-like ctenophore over one meter long Ctenoplana Large conical ctenophore that lacks tentacles flattened ctenophores that creep rather than swim Most ctenophores are bioluminescent at night 13-100 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 13-101 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Phylogeny of the Diploblasts Distinctions between diploblastic and tripoblastic conditions blurred Ctenophores and cnidarians have typical diploblastic characteristics 13-102 Due to recent detailed morphological studies and studies in gene expressions Gelatinous middle surrounded by outer epidermal layer derived from ectoderm Inner gut lining derived from endoderm However, the cells of the gelatinous layer are problematic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification If derived from endoderm 13-103 Organisms are characteristic of triploblasts But these cells may be derived from ectoderm Some refer to this layer as ectomesoderm Most cnidarians have few cells with the mesoglea, so little debate whether they are diploblastic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification However, during hydrozoan medusae stage Development of the entocodon layer has led to the suggestion that cnidarians are triploblastic One product of the entocodon is striated muscle Unlike contractile epitheliomuscular cells of other cnidarians Smooth and striated muscles are true muscle cells In triploblasts, true muscles are produced by mesodermal cells 13-104 Hydrozoans entocodon is ectodermal in origin Other smooth muscle cells present in hydrozoan medusae are of ectodermal origin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Probes of gene expression show that homologous gene expression is occurring in those of triploblast mesoderm and diploblast mesoderm. Intepretation of these probes is not complete 13-105 May represent independent origin of muscle in one branch of the diploblastic lineage Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Recent re-examination of the development of ctenophores has led to the observation that muscle cells in the middle layer originate from endodermal cells If this is confirmed 13-106 Ctenophores are triploblastic along with bilaterally symmetrical animals. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Body symmetry also debated Although adult ctenophore is radially symmetrical Cnidarian planula larva swims with one end moving forward This end could be designated as anterior end giving the planula a distinct anterior-posterior axis The question remains: Did the radially symmetrical cnidarians have a bilaterally symmetrical ancestor or does the genetic potential for bilateral symmetry predate the bilateral body plan? 13-107 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Given the yet unanswered questions to origins of development Branching order for the diploblastic phyla is not yet determined We predict a polytomy for cnidarian, ctenophoran, and placozoan branches 13-108 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Adaptive Diversification Neither phylum has deviated far from basic structural plan Cnidarians Achieved large numbers of individuals and species Demonstrate large diversity considering the simplicity of body plan Efficient predators Some feeding on prey larger than themselves Some feeding on small particles Some colonial forms grow to great size among corals Others show polymorphism and specialization of individuals within a colony 13-109 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Ctenophores Adhere to the arrangement of comb plates and biradial symmetry Vary in body shape and presence or absence of tentacles 13-110 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phylogeny and Adaptive Diversification Cnidarian Phylogeny 13-111 Relationships among cnidarian classes are still controversial: Which came first, the polyp or the medusa? One hypothesis postulates that the basal cnidarian was a trachyline-like hydrozoan with a medusa stage. Another hypothesis suggests that the basal cnidarian was an anthozoan polyp without a medusa in the life cycle.