1. Radiate Animals
C h a p t e r 1 3

2. A Fearsome Tiny Weapon: Things to know about Phylum Cnidaria
More highly organized than sponges
Relatively simple animals
Most are sessile
Some that are unattached (i.e.. Jellyfish) swim only feebly
Cannot chase their prey
Very effective predators that are able to kill and eat prey that are much more highly organized, swift, and intelligent with their tentacles
It uses it’s Nematocysts (– Tentacles that bristle with tiny, remarkably sophisticated weapons) that is endowed with potential energy to power it’s discharge.
When a nematocyst is fired, a tiny thread bursts from it achieving a velocity of 2 meters/second and an acceleration of 40,000 x gravity and instantly penetrated it’s prey injecting a paralyzing toxin.

3. More to know about Phylum Cnidaria:
Continued from previous slide:
Some nematocyst threads can penetrate human skin, causing sensations ranging from minor irritation to great pain, even death, depending on the species
Familiar cnidarians are sea anemones and jellyfishes; also some may know ctenophorans as comb jellies or sea walnuts
Contains more than 9000 species including: Branching plantlike hydroids; flowerlike sea anemones; horny corals (sea whips, sea fans, and others); and also stony corals who make up coral reefs and coral islands
Takes it’s name from cells called Cnidocytes
Most common type of cnida is the Nematocyst
Cnidocytes only formed by cnidarians
Ancient group with the longest fossil history of any metazoan reaching back more than 700 million years

4. Some ctenophores, molluscs, and flatworms eat hydroids bearing nematocysts, then store and use these stinging structures for their own defense
Widespread in marine habitats, and a few in freshwater
Most abundant in shallow marine habitats, especially in warm temperatures and tropical regions
No terrestrial species
Colonial hydroids are usually found attached to mollusc shells, rocks, wharves, and other animals in shallow coastal water, some species live at great depths
Floating and free-swimming medusae occur in open seas and lakes, often far from shore
Animals such as the Portuguese man-of-war have floats or sails by which the wind carries them
Quite efficient predators of organisms that are much swifter and more complex
Sometimes live symbiotically with other animals
Often as commensals on the shell or other surface of their host
Certain hydroids and sea anemones commonly live on snail shells inhabited by hermit crabs, providing the crabs some protection from predators

5. Algal cells frequently live as mutual's in the tissues of cnidarians
Some freshwater hydras and in reef-building have this too
Reefs are extremely important habitats for many other species of invertebrates and vertebrates in tropical waters
Many cnidarians have little economic importance, reef-building corals are an important exception
Fish and other animals associated with reefs provide substantial amounts of food for humans, and reefs provide an economic value of tourist attractions
Classes of Cnidaria
Hydrozoa – most variable class, including hydroids, fire corals, Portuguese man-of-war, and others
Cubozoa – including cube jellyfishes
Anthozoa – largest class including sea anemones, stony corals, soft corals, and others
Stauroza – has been proposed because recent phylogenies show that stauromedusans do not belong within the scyphozoa , do not make medusae but the polyp body is topped by a medusa-like region

6. Form and Function: dimorphism and polymorphism in cnidarians
All cnidarian forms fit into one of two morphological types (Dimorphism) :
Polyp - a hydroid form, which is adapted to a sedentary or sessile life
Medusa – a jellyfish form, which is adapted for a floating or free-swimming existence
Both seem very different, but actually each has retained the saclike body plan characteristic of the phylum
A medusa is essentially and unattached polyp with the tubular portion widened and flattened into a bell shape

7. Polyps Most have tubular bodies
A mouth surrounded by tentacles defines the oral end of the body
Mouth leads into a blind gut or Gastrovascular Cavity
Aboral end of the polyp is usually attached to a substratum by a pedal disc or other device
May reproduce asexually by budding, fission, or pedal laceration
Budding – a knob of tissue forms on the side of an existing polyp and develops a functional mouth and tentacles, if detached from the poly that made it, a clone is formed, If bud stays attached to the polyp that made it, a colony will form and food may be shared through a common gastrovascular cavity
When they do not bud, they are solitary
Others form clones or colonies
Distinction between colonies and clones is sometimes blurred when a colony fragments
Shared gastrovascular cavity permits polyp specialization
Polymorphism – colonies that include several morphologically distinct polyps with each specialized for a certain function, such as feeding, reproduction, or defense.
Hydranths – Feeding polyps
Gonangia – reproductive polyps, typically make medsae

8. Medusae Usually free swimming Have bell or umbrella-shaped bodies
Often exhibit tetramerous symmetry where body parts are arranged in fours
Mouth is usually centered on the concave side and may be pulled downward into frilly lobes that extend a long way beneath the umbrella or bell
Tentacles extend outward from the rim of the umbrella
Have sensory structures for orientation (or Statocysts) and light reception (ocelli)
Sensory info integrated with motor response by a nerve ring at the base of the bell; two such rings are present in hydrozoan medusae
Medusae of Class scyphozoa often called Scyphomedusae, where as those of class hydrozoa are Hydromedusae
Both differ from each other by the presence of a velum (or a shelflike fold of tissue from the bottom of the bell that extends into the bell
By reducing the cross-sectional area at the bottom of the bell, the velum increases the exit velocity of water from the bell, making each pulsation more efficient

9. Life Cycles
In cnidarian life cycle, polyp’s and medusae play different roles
Life cycle varies among cnidarian classes, but in general, a zygote develops into a motile planula larva
Planula settles on a hard surface and metamorphoses into a polyp
Polyp may make other polyps asexually, but eventually it produces free-swimming medusae by asexual reproduction
Polyps by budding, or other specialized methods like Strobilation
Medusae reproduce sexually and are dioecious
A cycle that contains both an attached polyp and a swimming medusa permits organisms to take advantage of both pelagic (open water) and benthic (bottom) environments
Occurs in true jellyfishes of class scyphozoa where the medusa is large and conspicuous and the polyps are typically very small
Most hydroids of class hydrozoa also feature a sessile polyp stage, often in colonies, and a pelagic medusa stage
There are many variations on the typical pattern

10. Continuation of Life Cycles….
Some hydrozoans have the polyp colony as not sessile but drifts across the ocean surface
The Portuguese man-of-war , Physalia, is one such drifter, using an inflated polyp as a gas-filled float
Other colonies are collections of both polyps and medusae where pulsating bells propel the colony through the water
Several life cycles do not include medusae
Anthrozoans are presumed to have diverged from an ancestor of the other cnidarians before the medusa evolved in the latter branch
Other cnidarians, including the hydrozoan Hydra, probably lost the medusa secondarily
Mechanism of loss is not clear in Hydra but in other hydrozoans a pattern of loss can be inferred from a comparison of modern forms
Most hydrozoans release medusae that later make gametes
A few forms make medusae without releasing them from the colony
Gametes then form in the gonads of the medusae retained by the polyp colony
Some species have only a short cuplike form surrounding the gonads
In other gonads develop right on the polyp colony with no trace of a medusa body
The latter organisms likely represent an extreme form of medusa retention and reduction

11. Cnidocytes
Each cnidocyte produces one of over 20 kinds of distinctive organelles called Cnidae that are discharged from the cell
During it’s development, a Cnidocyte is properly called a Cnidoblast
Once it’s cnida has been discharged, a cnidocyte is absorbed and replaced
Type of cnida: Nematocyst
Nematocyst is composed of material similar to chitin and containing a coiled tubular “Thread” or filament
Operculum – the little lid that covers the capsule on the top of a nematocyst
Some Cnidae have don’t have barbs or poison
Some spring and clasp their prey
Adhesive cnidae are used in attachment and locomotion, relying on the creature they are attached to
In Anthozoa, Cnidocytes are equipped with a trigger like cnidocil (- a modified cilium)
Anthozoan Cnidocytes have a somewhat different ciliary mechanoreceptor
Some sea anemones ,and perhaps other cnidarians, small organic molecules from the prey “tune” mechanoreceptors, making them sensitive to the frequency of vibration caused by prey swimming

12. Feeding and Digestion Polyps are typically carnivorous
They catch prey with their tentacles, passing them through the mouth into the gastrovascular cavity for digestion
In Hydra, the tentacles are hollow and the tentacle cavity communicates with the gastrovascular cavity
Polyps of a hydrozoan colony capture and digest prey extracellularly
It then passes through a digestive broth into the common gastrovascular cavity where intracellular digestion occurs
In Hydromedusae, both food type and digestive system are similar to that of the polyp
But the body is oriented with the mouth facing downward in the center of the bell; the mouth is at the end of a tube called the Manubrium
Scyphomedusae are typically lager than hydromedusae, but their basic form is similar
The mouth edge is extended as a Manubrium, often with four frilly oral arms sometimes called mouth lobes, used in capturing and ingesting prey
Anthozoan polyps, such as sea anemones, are carnivorous, feeding on fish or almost any animals of suitable size
A few species feed on minute forms caught by ciliary currents instead of eating large prey
Corals supplement their nutrition by collecting carbon from their algal symbionts

13. Class Hydrozoa: Hydroid colonies
Majority are marine and colonial in form
Typical life cycle includes both and asexual polyp and a sexual medusa stage (i.e. Obelia)
Typical hydroid consists of a base, a stalk, and one or more terminal zooids
The base by which colonial hydroids attach to the substratum is a rootlike stolon, or Hydrorhiza
Hydrocauli – one or more stalks that are given to rise
Tubular Coenosarc – the living cellular part of the hydrocaulus
Perisarc – a nonliving chitinous sheath that protectively covers the hydrocaulus
Hydranths – most zooids that are feeding polyps, also known as Gastrozooids
Thecate – the perisarc continues as a protective cup around the polyp into which it can withdraw for protection
Athecate – where the polyp is naked
Gonangium – a reproductive polyp from the medusae bud in Obelia
Entocodon – a unique derivative of ectoderm

15. Feeding, Digestion, and Epidermis
Basal – a pedal in which a slender stalk ends
Disc – part of said pedal, or Basal
Hypostome – the conical elevation in which the mouth is located
Nutritive-muscular cells – usually tall columnar cells and have laterally extended bases containing myofibrils
Hydrostatic skeleton – water in the gastrovascular cavity
Epitheliomuscular cells – compose most of the epidermis and serve both for covering and for muscular contraction
Interstitial cells – undifferentiated stem cells found among the bases of the epitheliomuscular cells
Gland cells – tall cells, located around the basal disc and mouth, that secrete an adhesive substance for attachment and sometimes a gas bubble for floating
Cnidocytes – occur throughout the epidermis
Sensory cells – scattered around the other epidermal cells, especially near the mouth and tentacles and on the basal disc
Nerve Cells – generally multipolar, although in more highly organized cnidarians the cells may be bipolar

16. Reproduction Reproduce sexually and asexually
In asexual reproduction, buds appear as out pocketings of the body wall and develop into young hydras that eventually detach from the parent
Most are dioecious
Temporary gonads usually appear in autumn
Testes or ovaries, when present, appear as rounded projections on the surface of the body
Eggs in the ovary usually mature one at a time and are fertilized by sperm shed into the water
Dactylozooids – fishing tentacles
Gonophores – a little more than sacs containing either ovaries or testes
Pneumatophore - a float believing to have expanded from the original larval polyp

17. Class Scyphozoa
Includes most of the larger jelly fishes, or Cup animals
May attain a bell diameter exceeding 2 meters and tentacles meters long, but most range from 2 to 40 cm in diameter
Most drift or swim in the open sea
Some even at depths of 3000 m.
Movement achieved by rhythmical pulsations of the bell
Bells differ in depth from a shallow saucer shape to a deep helmet or goblet shape, but a velum is never present
The margin of the umbrella that is scalloped, usually with each indentation, or notch bearing a pair of Lappets
Rhopalium – a sense organ
Nervous System - a nerve nest with a subumbrella net that controls bell pulsations and another, more diffuse net that controls local reactions such as feeding
Oral arms – used in capturing and ingesting prey

18. Class Scyphozoa
Gastric Pouches – gastrodermis extends down in little tentacle-like projections called Gastric Filaments
Radial Canals – a complex system that branches outward from the pouches to a Ring Canals in the margin and forms a part of the gastrovascular cavity
Scyphistoma – a hydralike form that may bud to produce a polyp clone
Strobilation – As process of the scyphistoma of Aurelia forming into a series of saucerlike buds
Ephyrae – saucerlike buds
Stobila – what's left of the saucer like buds

19. Class Cubozoa Once were considered an order of scyphozoa
Medusoid is the predominant form
Pedalium – a flattened, tough blade
Velarium – functions as a velum does in hydrozoan medusae, increasing swimming efficiency, but it differs structurally
Stings of some species can be fatal to humans
The polyp is tiny, solitary, and sessile

20. Class Anthozoa
Known as flower animals, they are polyps with a flowerlike appearance
There is no medusa stage
All marine and occur in both deep and shallow water and in polar seas as well as tropical seas
Hexacorallia – known as zoantharia contains sea anemones, hard corals, and others
Ceriantipatharia – containing only tube anemones and thorny corals
Octocorallia – also known as Alcyonaria containing soft and horny corals, such as sea fans, sea pens, sea pansies, and others

21. Sea Anemones Polyps are larger and heavier than hydrozoan polyps
Some are quite colorful
Cylindrical in form with a crown of tentacles arranged in one or more circles around the mouth of the flat oral disc
Pharynx – the part that the slit shaped mouth leads into
Siphonoglyph – extends into the pharynx
Monoexious species are Protandrous
Pedal laceration – by longitudinal fission, occasionally by transverse fission or by budding