Ch 8: Phylum Cnidaria

Medusa buaplan: Exhumbrella & subumbrella

*Ex*: Convex upper (aboral) surface. *Sub*: concave lower (oral) surface, contains mouth

Class Cubozoa (sea wasps and box jellies)

*Medusae nearly square* *Medusae w/ rhopalia and complex eyes* Very toxic sting Polyps and Medusa present

Class Scyphozoa (Jellyfishes) *Large w/ lots of color*

*Medusoid stage predominates, DUH* Polyps small, some groups lack polyps. Polyps produce medusae by *asexual budding* Polyp coelenteron divided by *4 mesenteries* *Medusae: w/out Velum, w/ thick mesoglea, distinct pigments*

Cnidocyte

*Nematocyte* (cnidae) found in cnidocyte Important in prey capture, defense, locomotion, attachment. SAME THING AS CNIDOCYST

Support in Cnidaria

*Polyps*: Hydrostatic 'skeleton', intake/outake of water maintains the structure, water in colenteron constrained by muscle fibers. *Medusae*: principal support is middle layer, a thin/flexible mesoglea or thick/stiff mesenchyme.

Class Anthozoa (Anemone, coral, sea pen)

-*W/out medusoid stage* -Cnidae both gastro-/epidermal -*Coelenteron divided by longitudinal mesentery* -Thick mesenchyme -Tentacles contain coelenteron -*Polyps reproduce a-/sexually* -Gametes in gastrodermis -Hermaphrodites -*NO JELLIES PRESENT*

Class Hydrozoa Man o' war

-Alternation of gen. -Colonial polyps, polymorph man o war -*Chitin* exoskeleton -*Coelenteron lacking pharynx & mesentery* -*Cnidae only in epidermis* -*Medusae w/out rhopalia,* no sensory elements

The cnidocyte is a SYNAPOMORPHY. It's a shared, derived character

...because it's found in all corals, anemones, etc.

Nervous system and muscles are more sophisticated in MEDUSA than POLYPS

...because medusa move while polyps do not.

Musculature is formed of epitheliomuscular cells

...it's derived from *ectoderm and endoderm*

Characteristics of Cnidaria

1. Multi-cellular 2. Diploblastic 3. Radial symmetry (contrast to sponges) 4. Cnidae 5. Musculature 6. Asexual polyploid & medusoid generations 7. Only body cavity: coelenteron 8. Without head, CNS, discrete gas exchange, excretory, circulatory structures 9. Nervous system: nerve net (np neurons) 10. Planula larvae (ciliated, motile, gastrula larvae)

Bauplan of Cnidaria

2 embryonic layers (diploblastic): endo/ectoderm. In between layers: mesoglea. Polyps and Medusae occur bc alternation of a-/sexual generations

Polyp bauplan: Physa "Dig in"

Aboral end of polyp. Rounded structure adapted for digging/anchoring in soft substrata. For burrowing anemones

Polypoid stage occurs in all 4 classes of cnidarians...

But greatly reduced in Scyphozoa and Cubozoa (b/c these are the jellyfish)

Massive calcareous skeleton for Cnidaria support

Calcium carbonate skeleton secreted by *epidermal cells*. The entire skeleton: *corallum*. Each separate polyp of the colony: *corallite* Ex: Elkhorn coral

4 classes of Cnidaria

Class Hydrozoa Class Anthozoa Class Cubozoa Class Scyphozoa

Layers of Cnidaria: OUTER EPIDERMIS

Consists of *epitheliomuscular* (myoepithelial) cells. Has sensory cells: *cnidocytes* Gland cells: produce mucus, has toxins. Interstitial cells (like archeocytes)

Myonemes in myoepithelial cells

Contain contractile myofibrils, can expand for movement. Interconnected with neighbor cells. Form longitudinal/circular sheets.

Parts of Rhopalium:

Cornea: surface Lens: concentrates light Retina: cells w/ pigments sensitive to light Has many *sensory* cells

Dactylozooid

Defense zooid on colony. Armed with cnidae, which has toxins

Polyp bauplan: Pharynx (the throat), siphonoglyphs present

Derived from ectoderm. Extends into the coelenteron (gastrovascular cavity). Bears 1+ ciliated grooves called *siphonoglyphs*, which drives water into gut cavity

Gastrozooid (type of zooid)

Feeding zooids. Capture, ingest prey on the colony. Bear tentacle and mouth

Medusa

Free, rarely benthic. Umbrella-shaped; unlike cylindrical polyps (b/c they move). Thick, jelly like mesoglea. Coelenteron present in central region of umbrella.

Layers of Cnidaria: MESOGLEA (Mesenchyme)

Gel-like. Thin or thick, with or without cells (clado-specific)

Polyp bauplan: Mesenterial filament

Inner edge of each mesentery armed w/ *cnidae, cilia, gland* cells. Give rise to long threads, *acontia* for defense/feeding

Gastrovascular cavity (COELENTERON)

It's the only body cavity. Saclike, partitioned (branched). Has a single opening, is both the mouth and anus

Cnidocyst

Large, complex intracellular structure. *Flask shape with sock that inverts when flask is opened*. Has *mechanoreceptor, Cnidocil* (looks like a thorn). Sometimes has *operculum, lid that opens*. When it's stimulated, the *tubule everts from capsule* & shoots out toxins. SAME THING AS CNIDOCYTE

Polyps bauplan: Pedal disc

Located at base (aboral end) of polyp. Used to attach to hard substrata, in most sea anemones. Produces mucus

Myoepithelial cells

Most *primitive muscle* cell in Metazoa. Columnar cells w/ *myonemes*: flat, basal extensions.

Nervous sensory cells (in epidermis)

Most primitive in animals. Naked, *nonpolar* mostly. Neurons arranged in reticular arrays (*nerve nets*). *Subepidermal*: between epidermis and mesenchyme (mesoglea). *Subgastrodermal*: between mesenchyme and gastrodermis

How medusas move:

Muscles around the umbrella (*coronal muscles*) provide jet propulsion

Colonial polyps

Polymorphic (can take diff forms), clade-dependent. There are thousands of zooids per colony. Type of zooids: 1. Gastrozooids 2. Dactylozooids 3. Gonozooids

Phylogenetics: the ancestor of the Cnidaria is the POLYP, and the MEDUSA IS DERIVED

Reasoning for a *polyp ancestor:* Anthozoa (anemones) most basal. Original body form is polyp. Medusa is derived. Metagenesis (alternation of gen) is derived.

Gonozooid

Reproductive polyp on colony. Produces medusae

Horny axial skeletons for Cnidarian support

Secreted by *amebocytes*. Hard skeleton, composed of *protein and polysaccharides* (the axial rod) Ex: black coral

Calcareous Sclerites for Cnidaria support

Secreted by *scleroblasts* in mesenchyme. The sclerites are what give coral their *color*/texture. Ex: pink coral

Relevence of Cnidaria

Serve as hosts to symbiotic small organisms (fish, parasites, shrimps). Coral mining in ancient times. Land formation. Anti-microbial/viral/cancer compounds (medicine)

Gastrovascular cavity a.k.a COELENTERON

Serves for circulation as well as digestion and distribution of food.

Layers of Cnidaria: INNER GASTRODERMIS

Similar to epidermis. Contains *nutritive-muscular cells* (type of myoepithelial). Also, *cnidocytes* and *gland cells* (mucus, toxins). Vacuoles for digestion

Cnidae

Stinging/adhesive structures. Each one resides in/produced by one cell called *cnidocyte*. Most common cnidae: *nematocysts*

Class Scyphozoa reproduce by STROBILATION

Strobilation: Produces medusae Repeated trasverse fission of scyphistoma (the form after planula larvae)

Polyp bauplan: Stolon Mneumonic: colon-y

The aboral end that arises from a common mat or stalk in *colonial forms*. Anchors the colony to the bottom of sea.

Polyp (some, anyway) bauplan: Mesenteries

The coelenteron is divided by mesenterie, projections of inner wall. a.k.a Septa. Can be in-/complete. Bear *mesenterial filament*. Also has *retractor muscles*

Rhopalium

The eye in *Cubozoa* Medusae (*box jellies*)

Cnidocyst discharge

The socket that is released shoots out w/ a pointed structure to penetrate prey. Proteins have toxins are liberated into prey. Cnidocyst: on tentacles of a jellyfish. Cnidocysts used to compete for space.

Polyps bauplan

Tubular structure: endoderm, mesoglea/mesenchyme, ectoderm, *inner gut sac (coelenteron)*. Large (m) to small (mm). Radial symmetry: bi-/quadriradial. Top axis: *oral* (mouth) Bottom axis: *aboral* (base, pedal disc, or Physa rounded structure)