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CHAPTER 13 – The Cnidarians (Radiate Animals)
Fig. 13.CO
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Phylum Cnidaria Hydra, jellyfish, coral, & sea anemones
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Jellyfish can be funny….
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General Characteristics
Radial symmetry Their body is arranged around an oral-aboral axis Oral end terminates in a mouth that is surrounded by tentacles Good for sessile; sedentary lifestyle Reach the tissue level of organization Cells are organized into groups of tissues that work together Contains 2 tissue layers (diploblastic) including: epidermis (gives rise to the outer body wall) and gastrodermis(gut)
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General Characteristics
Acoelomated organism- they contain NO body cavity All are carnivores Over 9,000 species are in the phylum No system for circulation, respiration or excretion-> occur by diffusion Some cnidarians can regenerate lost parts or even a complete body Most cnidarians are dioecious (2 forms: male and female) All named for the presence of cells called Cnidocytes: cells that contain the stinging organelle called nematocysts Found only in cnidarians
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General Characteristics
Diploblastic: contain 2 well-defined germ layers: ectoderm (epidermis) and endoderm(gastrodermis) Examples include: Hydras, jellyfish, sea anemone, corals, Portuguese man of war, box jelly fish Movement: mostly sessile, some move or “swim” slowly All contain some type of Gastrovascular cavity with one opening for food intake and elimination of waste
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General Characteristics
Both the outer (epidermis) and inner ( gastrodermis) contains nerve cells that are arranged in a loose network called a Nerve Net Innervates their primitive muscles that extend form the epidermal and gastrodermal cells Stimulus in one part will spread across the whole body by the network
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General Characteristics
Location Found mostly in marine habitats (some freshwater) Most abundant in warm (tropical), shallow, marine waters Colonial organisms can be found attached to rocks, animals, or wharves
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Ecological Role Predator/Prey Relationships
Neurotoxins in medical research Coral- jewelry, reef building Symbiosis with other organisms Coral reefs- habitat for many Great biodiversity Protect the coastline
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Symbiotic Relationships
Mutualism Hydroids and sea anemones can live on hermit crab shells Algal cells live in the tissue of cnidarians Clown fish and sea anemone Portuguese Man of War and the Nomeus fish
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Cnidarian Body Plans Dimorphism
Existence of 2 morphological types within the same species Allows the organism to obtain food from different locations All cnidarians fit into one of the following types: *polyp *medusa
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Cnidarian Body Plans Polyp (hydroid): Fits a sessile lifestyle
Can be found singly or in colonies Structure: tubular body with a mouth at one end (directed upward) surrounded by tentacles The aboral end is attached to a substrate
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Polyp Structure
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Cnidarian Body Plans Medusa Jellyfish form
Fits the swimming/floating lifestyle Mobile, move by weak contractions of body Bell or umbrella shape Mouth is usually directed downward and centered on the concave side and tentacles extend from the rim “mouth down” version of the polyp Contains more mesoglea (middle jelly-like layer) than polyp stage
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Polyp vs. Medusa
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Cnidocyte/Nematocyst
Cnidarians are named after the presence of cnidocytes: the stinging cell of cnidarians that produces the nematocyst Equivalent to the container(gun) that contains the stinging organelle (bullet) Nematocyst: the stinging organelle contained within the cnidocyte Most characteristic structure Helps with taxonomic classification Over 20 different types of nematocysts
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Structure of the Nematocyst
Enclosed within the cnidocyte (made of chitin) All cnidocytes (except for Anthozoa class) have a cnidocil- modified cilium that triggers the nematocyst to eject Tactile or chemical stimulation causes the nematocyst to discharge(prey swimming) After discharge the cnidocyte is absorbed and a new one replaces it
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Structure of the Nematocyst
Operculum: the covering (lid) that encloses the nematocyst inside the cnidocyte Triggered to open through a change in pressure Effects (types) of the nematocysts Barbs: (not found in all) Penetrants: inject poisons Volvents: Long tentacle (string like); entangles prey Glutinants: secretes an adhesive
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Steps of release for a nematocyst
Stimulation of the cnidocil The operculum opens When the operculum opens there is a change in pressure that forces the nematocyst out The nematocyst is ejected inside out Poisons may be injected when it penetrates the prey
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Cnidocyte: Before and After Discharge
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Nerve Net The simple nervous system of cnidarians
Variation of the nerve net is found between the classes Cnidarians have neurotransmitters on both sides of their synapses (junction between 2 nerve cells) this functions to transmits impulses in both directions Located in the gastrodermis and epidermis Forms 2 interconnected nerve nets
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Nerve Net They do form a simple neuromuscular system- their sensory nerve net plus they contain contractile fibers in their body wall to coordinate muscular contractions They do not have: Myelin covering their axons Central nervous system
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Nerve Net
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4 Classes of Cnidarians Class Hydrozoa (Hydra, PMOW, Obelia)
Class Scyphozoa (common jellyfish) Class Cubozoa (box jelly) Class Anthozoa (sea anemone and coral)
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Class Hydrozoa General Characteristics:
Mostly marine (except freshwater hydra) Most are colonial Most have a sexual (medusa) and asexual (polyp) lifecycle Examples we will discuss include: Freshwater hydra- only has a polyp stage Obelia PMOW
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Freshwater Hydra Belong to class Hydrozoa
Are solitary animals (not colonial) Found in freshwater environments attached to anything…rocks, aquatic leaves… Only found in the polyp stage (no medusa stage) Feed mainly on plankton
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Body Plan of the Hydra Cylindrical tube with a point of attachment at the bottom and mouth at the top of the tube surrounded by tentacles Contains the following parts Basal disc Located at the bottom of the tube body Serves as the site for attachment Secretes substances for adhesion Can produce gas bubble at the end of the basal disc for floating
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Body Plan of the Hydra Hypostome Mouth Gastrovascular cavity
Cone shaped elevation Mouth is at the center Surrounded by tentacles Mouth Ingests food Gastrovascular cavity Open cavity that is surrounded by the body wall Site of extracellular digestion
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Body plan of the Hydra
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Body wall of the Hydra Surrounds the GVC Consists of 3 layers
Epidermis (outer layer) Gastrodermis (inner layer) Mesoglea (middle layer)
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Epidermis of the Hydra Outer layer that serves as protection
Composed of several cell types Epitheliomuscular cells Interstitial cells Gland cells Nerve cells Sensory cells Cnidocytes
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Epitheliomuscular Cells
Composes the majority of the epidermis Functions to protect the hydra and to produce muscular contractions
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Interstitial Cells Located at the base of the epitheliomuscular cells
Functions as stem cells to turn into almost any other cell type (cnidoblasts, sex cells, buds, nerve cells) ***What type of sponge cell does this sound like?
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Gland Cells Located on the basal disc and around the mouth
Functions to secrete adhesive or lubrication
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Cnidocytes Contain nematocysts
Function to protect the organism and to trap and kill food
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Sensory Cells Located around the mouth, tentacles, or basal disc
Allows the organism to be stimulated by the environment and to be aware of its surroundings Structure: one end had flagella for stimulation and the other end goes to the nerve cells
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Nerve Cells Connects with sensory cells, other nerve cells, cnidocytes, or muscular cells Coordinates all of the activities
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Body wall of the Hydra
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Gastrodermis of the Hydra
Lines and directly touches the GVC The following cell types are found in the gastrodermis: Nutritive-muscular cells Interstitial cells (performs the same function as in the epidermis- they act as stem cells) Gland cells
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Nutritive-muscular cells
Contain cilia to create a flow of food, water and nutrients inside the GVC Tall cells that contain food vacuoles Absorbs, digests and circulates food and fluids Site of intracellular digestion
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Gland cells Gland cells in the gastrodermis are located around the hypostome and inside the column Function: Hypostome gland cells secrete lubricant and digestive enzymes into the GVC Inside the GVC the gland cells secrete digestive enzymes
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Gastro-vascular cavity
Open space inside the cnidarian Filled with water to act as a hydrostatic skeleton (provides support) Lined with gland cells to help with digestion Extracellular digestion occurs in the GVC
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Mesoglea Location: in between the epidermis and gastrodermis
Structure: non cellular, gelatinous material composed mainly of water Function: support and flexibility Mesoglea is thickest in the stalk (strength) Mesoglea is thinnest in the tentacles (flexibility)
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Locomotion of Hydra The Hydra can move in 3 ways:
Glide on basal disc (bottom of the organism) Bend over and attach tentacles to the surface Gas bubble
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Feeding and Digestion: Hydra
Hydras eat: crustaceans, insect larvae, annelid worms Feeding process: Tentacles trap prey with the help of the nematocyst (encased in the cnidocyte) Mouth widens (with the help of gland cells) to engulf prey Extracellular digestion takes place in the GVC with the help of gland cells secreting digestive enzymes Food particles go into nutritive-muscular cells for further intracellular digestion
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Glutathione A chemical activator that triggers the feeding process of certain cnidarians Cause the tentacles to move more Causes the hydra to be more alert Causes the gland cells to release lubricant and widen the mouth Prepare for food intake
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Hydra Reproduction Reproduce both sexually and asexually
Sexual Reproduction Form temporary gonads in the fall Eggs and sperm and are shed into the water and form fertilized eggs Hydra will hatch in the spring (favorable environment) Asexual Reproduction Budding: starts as a growth on the side of the “parent” hydra Eventually will detach from parent and live on its own
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Hydra Budding
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Obelia- Hydroid colony
2nd example of Hydrozoan class Has a polyp and medusa stage in its life cycle All polyps in the colony are usually interconnected Contains a base, stalk and terminal zooids Zooid: general term for an individual polyp animal in a colonial cnidaria
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Obelia Structure Hydrorhiza: (root-like) base that attaches the colonial polyp to a substrate Hydrocauli: (stem) part that extends from the hydrorhiza and the individual zooids Zooid: attached to the hydrocaulus; individual polyp animals
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Obelia Structure
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Types of tissue found in the Obelia
Coenosarc: the inner, living, cellular part of the Obelia Perisarc: the outer, protective, non-living, covering of the Obelia Made of chitin
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Obelia Structure
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Types of Zooids Gastrozooids Also called hydranths
Feeding animals of the colony All have tentacles with cnidocytes Feed on crustaceans, worms, and larvae If one polyp eats it provides nutrients for the entire colony Digestion and circulation of food is aided by ciliated nutritive muscular cells and body wall contractions
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Types of Zooids Gonozooids Reproductive polyp that release medusa buds
Obelia reproduces both sexually and asexually. Sexual Reproduction: (most common pathway) * young medusa leaves the colony * medusa buds mature and shed gametes (sperm and egg) * fertilization occurs * Zygote created turns into a Planula (free swimming larvae stage) that swims and attaches to form new colony
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Obelia Reproduction Asexual Reproduction:
Occurs by budding-> increases the size of the colony Occurs but outgrowth of the body wall Creates new gastro- and gono- zooids
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Obelia Reproduction
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Medusa structure of Obelia
“Jellyfish” form Contains: Velum: membrane under the surface if the umbrella of the medusa that projects inward Contains tentacles with cnidocytes Manubrium: tissue projecting from the oral side of the medusa; surrounds the mouth
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Internal medusa structure of Obelia
Entire system is lined by the gastrodermis Gastrovascular cavity is continuous from the mouth to the tentacles Pathway of food flow: (disperses food w/o circulatory system) Mouth-> opens to the manubrium projecting form the oral side of the mouth-> leads to the stomach and the 4 radial canals-> connects to the ring around the margin of the bell-> connects the tentacles
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Movement of Obelia medusa
Aboral side first Characterized by weak jet propulsion Muscular pulsations fill bell with water and empty water to propel medusa
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Obelia: Medusa Stage
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Nerve Net of Obelia Medusa
Concentrated in 2 nerve rings at the base of velum Composed of Statocysts: functions in equilibrium Ocelli: light sensitivity
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Nerve Net
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Physalia physalis Portuguese Man of War (PMOW)
3rd example of Hydrozoan class Located in warmer, tropical waters Considered to be a polymorphic swimming community that is composed of several types of animals that swim and act as 1 colony Nematocysts are located all over the tentacles Some are very dangerous-> secrete a neurotoxin
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Structure of PMOW Pneumatophore Rainbow colored float Filled with gas
Carries future generations and acts as a nursing center Can briefly deflate for protection
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Types of individuals in a colony
Gastrozooids: feeding zooids that ingest the food Dactylzooids: long fishing tentacles that sting, capture the prey and bring it to the mouth On average are 30 feet long (can be as long as 165 feet underwater) Covered in nematocysts Gonozooids: sacs with ovaries and testes Function in reproduction
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Portuguese Man 0’ War Tentacles of Physalia physalis
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