Radiates

Phylum Cnidaria and Phylum Ctenophora

General Characteristics or both groups:

1 - radial or biradial symmetry. No definitive head

2 - tissue level of organization - although a few organs occur

3 - two germ layers present:

4 - Body wall -

5 - internal body cavity = gastrovascular cavity lined by gastrodermis

6 - extracelluar digestion

7 - Most have tentacles

8 - first to possess true nerve cells - nerve net or plexus of nerve cells.

No CNS

9 - first to possess sense organs

Cnidarians

Four classes are recognized:

Class Hydrozoa - hydroids, fire corals, and Portuguese man-of-war ...

Class Scyphozoa - "true jellyfishes"

Class Cubozoa - cube jellyfishes

Class Anthozoa - sea anemones, stony corals, soft corals ...

Polyp and Medusa stages

saclike body plan

Nematocysts -

Cnidocyte -

Class Hydrozoa

Most Colonial; most have polyp and medusa stages

Polyp stage - cylindrical tube; basal disc; Gland cells; mouth w/ tentacles

Epidermis- many cell types: (gland cells, cnidocytes, nerve cells)

1- epitheliomuscular cells -

Gastrodermis -

Mesoglea -

Hydra - solitary hydrozoan

Obelia - colonial hydrozoan

Medusa Stage of Hydrozoans differ from medusa of Scyphozoans: velum

Portuguese man-of-war - floating colony of polyps and medusas, not a medusa stage (jellyfish).

Pneumatophores

Class Scyphozoa (true jellyfish)

Mesoglea -

Gastrovascular cavity = Four gastric pouches with gastric filaments - radial canals - ring canals

Class Anthozoa - "flower animals" sea anemones, hard corals, sea fans, soft corals......

No medusa stage

Solitary or colonial

Oral disc with tentacles - pharynx - gastrovascular cavity with septa (mesenteries) - increase surface area of cavity

Acontia with nematocysts

Hard Corals - Zoantharians

Calcareous cups - for protection; exoskeleton; living corals form sheet of tissue over exoskeleton so all polyps are connected.

Alcyonarian Corals - Coral reefs; most productive of all ecosystems

Large formations of calcium carbonate laid down over 1000's of years.

Phylum Ctenophora - sea walnuts, comb jellies

eight rows of comb-like plates

no nematocysts (one exception)

Most free swimming but at mercy of tides and currents.