Cnidaria

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Phylum Cnidaria

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A group of more than 9000 species. Has radial or bi radial symmetry. Radial symmetry, in which body parts are arranged concentrically around an oral-aboral axis, is particularly suitable for sessile or sedentary animals and for free-floating animals because they approach their environment or it approaches them from all sides equally.

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360 degrees symmetry

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Bi radial symmetry

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A type of radial symmetry in which only two planes through the oral-aboral axis divide the animal into mirror images because the body contains a paired structure.

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Mirror image symmetry

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Cnidocytes

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Contains the stinging organelles (cnidae) characteristic of the phylum.

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Stinging organelles

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Nematocysts

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Cnidae come in several types, including nematocysts. Nematocysts are formed only by cnidarians.

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Stinging organelles

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Cnidarians part 2

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Ancient group with the longest fossil history of any animal phylum, reaching back more than 700 million years. Although their organization has a structural and functional simplicity. They mostly live in freshwater.

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Polyp

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Hydroid form, which has a sedentary or sessile lifestyle

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Hydra form

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Medusa

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Jellyfish form, which has a floating or free-swimming existence

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Jellyfish

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Budding

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A knob of tissue forms on the side of an existing polyp and develops a functional mouth and tentacles. A bud that detatches from the polyp is a clone.

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Clone formed from original

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Fission

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One-half of a polyp pulls away from the other, or by laceration of the pedal disc, in which tissue torn from the pedal disc develops into tiny new polyps.

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One polyp tears away from another polyp

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Medusae

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Free-swimming and have bell shaped or umbrella shaped bodies and tetramerous symmetry (body parts arranged in fours). The mouth is usually centered on the concave side, and the tentacles extend from the rim of the umbrella.

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Sac-like bodies and tentacles

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Polyp and Medusa

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Possess two tissue layers with an extra cellular matrix (ECM) called mesoglea between them. Mesoglea is much thicker in a medusa, forming the bulk of the animal and making it more buoyant. Because of the mass of mesoglea ("jelly"), Medusa are referred to as jellyfish.

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Mesoglea, sac-like bodies, tentacles, jellies not fish

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Locomotion

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Colonial polyps are permanently attached. Hydras can move freely across a substrate by gliding on their pedal disc, aided by mucous secretions. Sea anemones can move similarly on their pedal discs. Hydras can also use a "measuring worm" movement, looping along a surface by bending over and attaching their tentacles to the substratum. They might detatch or turn hand springs and form a gas bubble on the pedal disc to float to the surface.

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Movement

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Planula

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A zygote develops into a free-swimming larva

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Free-swimming larva.

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Gastrovascular cavity

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Body cavity in certain lower invertebrates that functions in both digestion and circulation and has a single opening serving as both mouth and anus

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Interacts with the tentacles

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Epidermis

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Outer wall (ectodermal) surrounding the gastrovascular cavity

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Outer wall

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Gastrodermis

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Inner wall of gastrovascular cavity (endodermal)

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Inner wall 😐

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Mesoglea

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Lies between the epidermis and gastrodermis and adheres to both layers. It is a gelatinous ECM without fibers or cellular elements in hydrozoan polyps. It is thicker in medusae and has elastic fibers; in scyphozoan medusae, it has ameboid cells.

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Gastrodermis

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The gastrodermis, a layer of cells lining the gastrovascular cavity, consists chiefly of large, ciliated columnar epithelial cells with irregular flat bases.

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Hydrostatic Skeleton

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A mass of fluid or plastic parenchyma enclosed within a muscular wall to provide the support necessary for antagonistic muscle action.

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Epidermis (part 2)

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The epidermal layer containing epitheliomuscular, interstitial, gland, cnidocyte, and sensory and nerve cells.

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Epitheliomuscular cells

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Forms most of the epidermis and serves both for covering and for muscular contraction.

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Interstitial cells

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Undifferentiated stem cells found among the bases of the epitheliomuscular cells. Differentiation of interstitial cells produces cnidoblasts, sex cells, buds, nerve cells, and others which are generally not epitheliomuscular cells.

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Gland cells

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Particularly abundant around the mouth and in the pedal disc of hydra. They secrete mucus or adhesive material.

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Cnidocytes

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(Containing cnidae) occur throughout the epidermis. They may be between the epitheliomuscular cells or housed in invaginations of these cells, and are most abundant in the tentacles.

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Sensory cells

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Scattered among the other epidermal cells, especially around the mouth and tentacles. The free end of each sensory cell bears a flagellum, which is the sensory receptor for chemical and tactile stimuli. The other end branches into fine processes, which synapse with nerve cells.

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Nerve cells

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Nerve cells are often multi polar (have many processes), although in more highly organized cnidarians, the cells may be bipolar (having two processes). Their processes (axons) form synapses with sensory cells and other nerve cells, and junctions with epitheliomuscular cells and cnidocytes. Both one-way and two-way synapses with other nerve cells are present.

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Cnidae

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Tiny capsules contained within cnidocytes

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Operculum

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Gill covering in bony fishes

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Nematocyst

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Stinging organelle of cnidarians

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Cnidoblast

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During it's development a cnidocyte is called a cnidoblast

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Cnidocil

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A modified cilium

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Nerve Net

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This plexus of nerve cells occurs both at the base of the epidermis and at the base of the gastrodermis, forming two interconnected nerve nets.

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Class Hydrozoa

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Most are Marine and colonial in form, and the typical life cycle includes both an asexual polyp and a sexual Medusa stage.

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Hydra: A freshwater Hydrozoan

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The common freshwater hydra is a solitary polyp and one of the few cnidarians found in fresh water.

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Hydroid Colonies

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Another part of Class Hydrozoa that have a Medusa stage in their life cycle.

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Hydranths

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Like Hydra, they capture and ingest prey. Digestive broth passes into a gastrovascular cavity where intracellular digestion occurs.

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Hydranths (part 2)

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In contrast to hydras, new individual's that bud so not set arch from the parent; thus the size of the colony increases. New polyps may be hydranths or reproductive polyps called gonangia. Medusae are produced by budding within the gonangia. Young medusae leave the colony as free-swimming individuals that mature and produce gametes (sperm and eggs).

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Velum

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A membrane on the subumbrellar surface of jelly fishes of class Hydrozoa.

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Manubrium

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The portion projecting from the oral side of a jellyfish medusa, bearing the mouth; oral cone; presternum or anterior part of sternum.

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Statocysts

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Small organs of equilibrium.

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Ocelli

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Light-sensitive organs

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Other hydrozoans

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Siphonophores are hydrozoans that form floating colonies and include Physalia.

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Portuguese man-of-war

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Physalia physalis (order Siphonophora, class Hydrozoa). Colonies often drift onto southern ocean beaches. Each colony of Medusa and polyp types is integrated to act as one individual. As many as a thousand zooids may be found in one colony. The nematocysts secrete a powerful neurotoxin.

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Hydrocorals

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Certain members of the cnidarian class Hydrozoa that secrete calcium carbonate, resembling true corals.

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Class Scyphozoa

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Includes most of the larger jellies, or "cup animals". The medusae of scyphozoans, have no velum. Many have a scalloped margin, each notch bearing a sense organ called a rhopalium and a pair of lobe-like projections called lappets.

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Class Staurozoa

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Commonly called stauromedusans. Life cycle does not contain a Medusa stage.

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Class Cubozoa

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The Medusa is the predominant form; the polyp is inconspicuous. The base of each tentacle is differentiated into a flattened, tough blade called a pedalium. The umbrella margin is not scalloped, and the subumbrella edge turns inward to form a velarium.

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Class Anthozoa

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"Flower animals", are polyps with a flower like appearance. There is no Medusa stage. All are Marine and occur both in deep and shallow water, in polar and tropical seas as well.

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Subclass Zoantharia

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(Hexacorallia) composed of sea anemones, hard corals, and others.

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Ceriantipatharia

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Includes only tube anemones and thorny corals.

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Octocorallia

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(Alcyonaria) containing soft mad horny corals, such as sea fans, sea pens, sea pansies, and others. Octocorallians are octomerous (built on a plan of eight)

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Sea Anemones

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Larger and heavier than hydrozoan polyps. Most range from 5mm or less to 100 mm in diameter. Sea Anemones are cylindrical, with a crown of tentacles arranged in one or more circles around the mouth on the flat oral disc.

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Sea Anemones (part 2)

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The slit-shaped mouth leads into a pharynx. A siphonoglyph, extends into the pharynx. Acontia are lower ends of septal edges that are prolonged into threads.

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Zoantharian Corals

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Belong to the order Scleractinia, of the subclass Zoantharia. Sometimes called stony or true corals. Arranged in multiples of six (hexamerous), hollow tentacles surround its mouth.

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Zoantharian Corals (cont.)

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Instead of a pedal disc, the epidermis at the base secretes a limy skeletal cup.

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Octocorallian Corals

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Includes soft corals, sea pansies, sea pens, sea fans and other gorgonian corals (horny corals). They have strict octomerous symmetry, eight pinnate tentacles and eight unpaired, complete septa.

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Coral Reefs

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Large formations of calcium carbonate (limestone) in shallow tropical seas. Coral bleaching where corals become white and brittle after expelling zooxanthellae.