Phylum

Cnidaria

Phylum Cnidaria

• jellyfish, hydras, sea anemones, corals

• carnivorous• radial symmetry• have stinging cells called

cnidocytes which contain nematocysts

• nematocysts are poison filled harpoons used for catching prey

Radial Symmetry Bilateral Symmetry

Planes ofsymmetry

Plane ofsymmetry

Ventral side

Dorsal side

Posterior end

Anterior end

 Body Symmetry

Cnidarian Body Plan

• radial symmetry with tentacles• two stages to life cycle

– Polyp– Medusa

• body surrounds an internal space, gastrovascular cavity

• gastrovascular cavity lined with gastroderm (tissue layer)

• epidermis is outermost layer• mesoglea between gastroderm and epidermis

(jelly-like)

Polyp

• Sessile

• cylindrical body

• mouth faces upwards

• tentacles point up

Medusa

• Motile

• bell-shaped body

• mouth underneath

• tentacles point down

Epidermis

Mesoglea

Gastroderm

Mesoglea

Gastrovascular cavity

Mouth/anus

Tentacles

Tentacles

Mouth/anus

Gastrovascularcavity

Polyp

Medusa

 The Polyp and Medusa Stages

Feeding

• prey is paralyzed and pulled into the gastrovascular cavity

• digestion is extracellular

• nutrients are absorbed by gastroderm

• nutrients dispersed by diffusion

• anything not digested leaves through the mouth/anus

Respiration, Circulation, Excretion

• all are done by diffusion through the body wall

Response

• specialized cells of the nerve net receive stimuli

• nerve net is evenly distributed throughout the body

• no true brain

• also have cells to determine light intensity and gravity

Movement

• done by hydrostatic skeleton in polyps

• done by jet propulsion in medusa (like a water pump)

Reproduction: Sexual

• fertilization is external

• separate male and female organisms

• zygote forms a free-swimming larva

• larva grows into a new polyp

• polyp then buds to form new medusa

Reproduction: Asexual

• polyps reproduce by budding

• new polyps may grow off the side of existing polyps

• polyp may produce medusas

Fertilization occurs in the open water, producing many diploid zygotes.

Sperm (N)

Egg (N)

Haploid

Diploid

FERTILIZATION

MEIOSIS

Male medusa(2N)

Zygote (2N)

Polyp

Buddingpolyp

Youngmedusa

Female medusa(2N)

Each zygote grows into a ciliated larva. The larva eventually attaches to a hard surface and develops into a polyp.

Swimming larvaThe polypbuds to release young medusas.

Adult medusas reproduce sexually by releasing gametes intothe water.

Jellyfish Life Cycle

Jellyfish

• class Scyphozoa (cup animal)

• life is spent mostly in medusa stage

• larva and polyp stage are usually the same

• reproduce sexually

Hydras

• class Hydrozoa• usually grow in

colonies• no medusa stage in

life cycle of freshwater hydras

• may reproduce either sexually or asexually

Anemones and Corals

• class Anthozoa (flower animals)

• only have a polyp stage in their life cycle

• many are colonial• reproduce sexually

(some asexually)• larva forms a new

polyp