Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 13

Radiate Animals

Position in Animal KingdomBoth phyla Cnidaria and Ctenophora make up the radiate animals. Biradial symmetry is radial symmetry limited to two planes that create mirror images.Other eumetazoans have bilateral symmetry or their radial symmetry is derived from a bilateral ancestor.Neither Cnidaria nor Ctenophora have advanced beyond tissue level of organization although a few organs are seen.

Biological ContributionsBoth phyla have two well-defined germ layers: ectoderm and endoderm; mesoderm may be derived from ectoderm.There is an internal body cavity: the gastrovascular cavity.Extracellular digestion occurs in the gastrovascular cavity; gastrodermal cells accomplish cellular digestion.

Biological ContributionsMost have tentacles, which are extensible projections for food capture.Radiates are the simplest animals with nerve cells; there is no central nervous system.Radiates are the simplest animals with sense organs: statocysts and ocelli.Locomotion is by muscular contraction or ciliarycomb plates.The polyp and medusa forms allow wider ecological possibilities.

Definition The Cnidaria are tentacle-bearing Metazoa, of the tissue grade of construction, with primary radial symmetry, composed essentially of two epithelia with a gelatinous matrix (mesoglea) between them, with nematocysts and with only one internal cavity, the digestive cavity, with a mouth, but no anus.

Characteristics

Entirely aquatic, some in fresh water, but mostly marine.Primary radial symmetry around a longitudinal axis with oral and aboralends; No definite head

CharacteristicsTwo basic types of individuals: polyps and medusae.

Figure 13-2

CharacteristicsSpecial cells, cnidocytes (from which the phylum gets its name), with stinging organelles, nematocysts, in either epidermis, gastrodermis or both.A primitive nervous system consisting of a nerve net at the base of the epidermaland gastrodermal layers, with impulse transmission tending to be radiating and with typically nonpolarized synaptic junctions.

CharacteristicsMost feed on zooplankton, although some utilize larger animals and some are suspension feeders on fine particulate matter. Prey is caught, immobilized and killed with the nematocysts on the tentacles and digestion is initially extracellular, then intracellular.

CharacteristicsAsexual reproduction by budding (in polyps) or sexual reproduction by gametes (in all medusaeand some polyps). Sexual forms monoecious or dioecious; Primitive gonads; Ciliated free-swimming planula larva in life cycle of most. (Image is from the Biodidac image data base maintained at the university of Ottawa.)No respiratory or excretory system and no coelomic cavity.

Form and FunctionPolymorphism (poly = many; Morph = form). Many members of phylum display dimorphism and polymorphism.Two major morphological types are polyp and medusa, but there can be many different types of polyp in a colony creating polymorphic colonies.

Basic Body FormsPolyp = hydroid form, adapted for sedentaryor sessile life. Tubular body w/ mouth at one end surrounded by tentacles. Aboral end usually attached to a substratum by a pedal disc or by skeletal secretion or by stolons(root-like outgrowths).

Figure 13-1 B

Basic Body Forms

May live singly or in colonies. Colonies of some species w/ more than one kind of individual (zooid or "person"), each specialized for a certain function, e.G. Feeding, reproduction or defense.

Colonial Hydroids (Class Hydrozoa)

Usually have life cycles featuring both polyp and medusa stages - can take advantage of feeding in a bottom environment as a polyp and in the open water as a medusa.

Complex Hydrozoan Colonies

The Portuguese Man-o-war is actually a polymorphic colony of individual zooids serving differing functions.

Basic Body FormsMedusa = jellyfish form, free swimming and sexually mature form,

Bell-shaped or umbrella-shaped bodies, tetramerous symmetry, Mouth centered on concave side, tentacles and one or more types of sense organs borne on rim of umbrella.

Physalia from Jamaica

Basic Body FormsMedusa provides dispersal mechanism offspring don’t compete with parents. Mesoglea much thicker than in polyp -constitutes bulk of animal - makes it buoyant = "jelly" of jellyfish.

Nematocysts

Functions:Hold prey.Provide adhesive which aids in locomotion by attaching tentacles to substrate.Penetrate and anchor in prey and poison it.

Figure 13-3

cnidocil

Nematocysts• Chemical and

mechanical stimuli• Discharge 2 m/sec• Penetrates most

biological substances • Tips open, closed,

some wrap• Contents injected,

adhesives, toxins

Nematocyst PoisonMost dangerous to man is Cubozoan jellyfish, the sea wasp (Chironexfleckeri). Caused more human suffering and death off Australian coasts than Physalia (Portuguese man-of-war) has in any of its home waters. Symptoms range from burning pain at site of contact with tentacles through skin lesions and eruptions of various sorts, often severe enough to leave scars, to generate great pain, fever, prostration, and respiratory interference. Most severe rxns due to shock = allergic rxn. "Sting" of most is imperceptible to humans. Problems usually occur only when repeatedly stung.

the sea wasp, Chironex fleckeri -a box jellyfish (Cubozoa)…

…and one of the deadliestcreatures known!

a box jelly baby

http://www.ucmp.berkeley.edu/cnidaria/Chironex.html

Nervous System

First animals to possess one, so of much research interest. Diffuse system = nerve net. Conduction radiates. Synaptic vesicles on both sides of many synapses, so bi-directional.

Sense Organs / Statocyst

Balance organ.Ball of sandy or stony material shifts when animal moves.It stimulates surrounding neurons and provides animal with information about its position. caspar.bgsu.edu/.../Neuroethology/ labs/images/Statocyst.jpg.

Sense Organs /Ocelli and Rhopalium

Significant that first organs are for perception of external world. Ocellus is first primitive light-sensing organ.Usually found on bell of scyphozoan and Cubozoan medusae. Often combined with statocyst into rhopalium.

Class Hydrozoa

700 species, majority marine and colonial, a few fresh water (F.W.) Species (the only F.W. Cnidarians).Typical life cycle includes both asexual polyp and sexual medusa (some with no polyp and some with no medusa). Includes hydrawhich is not typical, but much studied because easy to get and maintain.

Class HydrozoaColonies with polymorphism very common (e.G. Hydractinia) and Physalia. What is adaptive value of colonies? If can’t specialize body areas into organs (no true mesoderm), specialize individuals in a colony. Get “superorganism”.

Class Hydrozoa – Life CyclesThere is general trend to reduced medusa stage and increased colonialism and polymorphism.In the previous scheme, the medusa is the most primitive form because it is sexual and therefore represents an adult form.Polyp is larval stage. Budding produced more polyps asexually. The budding is equivalent to formation of medusa buds in present polyps.

Class Hydrozoa – Life Cycles

With the evolution of polyp, some groups reduced the medusa. First it remained attached, then the polyp epidermis itself produced the gametes.

Scyphozoa, Cubozoa and Anthozoa

Jellyfishes (classes Scyphozoa and Cubozoa) are predominantly medusae w/ polyp stage very reduced or completely absent. Corals and anemones (class Anthozoa) are entirely polyps, no medusa.

Class Scyphozoa (the Common Jellyfishes)

Pelagic cnidarians in which the medusa is the dominant and conspicuous form. Often seen in coastal waters and usually feared by local swimmers. Only about 200 species, but very abundant, so play important ecological role. Found everywhere in oceans, extending to depths of 3000 m or more.

Class Scyphozoa

Larger and more brightly colored than hydromedusae. Compensate for increased size by increasing complexity of gastrovascular system (oral arms and radial canals) and by better control of swimming (no velum around bell).

Class Scyphozoa-Life CycleIn some species the scyphistoma transforms directly into a young medusa, (additional evidence that the polyploid form was derived from a larval stage in the evolution of the cnidarians, or evidence that scyphozoans evolved from anthozoans?). In most species of scyphozoans young medusae are budded off transversely from the oral end of the scyphistoma which has become a strobila.

Class Cubozoa.

Formerly classified as order in Scyphozoa. Cuboidal medusa with tentacles hanging from corners of medusa. Polyps very small and most often unknown. Active swimmers and feeders in warm tropical waters. Many poisonous (see sea wasp above and in text).

Class Cubozoa

It is interesting to note that the margin of the bell of the cubozoan medusa has a fold of tissue analogous, but not homologous, to the velum of hydrozoans.

Class Anthozoa

Either solitary or colonial polypoidcnidarians in which the medusa stage is completely absent.Include sea anemones, corals, sea fans and sea pansies. Largest Cnidarian class with over 6000 species.

Class Anthozoa-Polyp StructureMore complex than hydrozoan polyps.Pharynx extends more than half way into the gastrovascular cavity. Mesenteries = longitudinal sheets that divide the gastrovascular cavity into radiating compartments. The edges of the mesenteries bear nematocysts. May be for internal gas exchange.

Class Anthozoa-Polyp StructureMany traits in common with Scyphozoa(cellular mesoglea, gastrovascularcavity divided into compartments, cnidocytes in gastrovascular cavity and gastrodermal gonads) so probably more closely related to them than to hydrozoa.

Anthozoa

Anemones have no secreted skeleton, are solitary and tend to be much larger than the corals. Corals are colonial and are often of extreme ecological importance because of their reef building.

-> prey killed and pushed into gut• gastrodermal cells inject digestive enzymes• adjacent cells absorb food or• in colonies, moved in a slurry throughout

plankton - Oahu, Hawai’i

Bi-radial Symmetry Bioluminescence in Paraphyllina intermedia (Scyphozoa)

http://www.biolum.org/marine/biolum2/middle/livinglights/lljellycontent.html

Scyphozoan MedusaColonial Hydroid colony and Montastrea annularis polyps

Fire Coral and Tube Anemone

http://ag.arizona.edu/~dgalbrai/jelly.gif

a NW hydromedusa (Hydrozoa)

Phylum Ctenophora

A minor groupFewer than 100 species

All are marineHave 8 rows of comb-like plates for locomotionNearly all free-swimmingMany are bioluminescent

Compare to CnidariaSimilarities with Cnidaria

the “coelenterates”mesogleasymmetry – biradial and radialTissue Grade of Development

Differences:gastrulation -ctenes - plates of fused ciliarows of ctenes = costaeDo not have nematocysts (one exception)

a benthic Ctenophore! (platyctene)…

…on a starfish arm!(Asteropsis carnifera)

Kailua-Kona, Hawai’i

The End.