BIO 385 Invertebrate Zoology
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Invertebrate Diversity
Phylum Cnidaria Cnidarians

(Click on any image for larger version)

Cnidarian Characteristics

  • Diploblastic; endoderm & ectoderm separated by mesoglea
  • Radial symmetry
  • Presence of cnidae/nematocysts in cnidocytes (stinging cells) (see photo at right)
  • Alternation of polyploid (polyp) and medusoid generations
  • Gastrovascular cavity with single mouth/anus
  • Tissue grade organization
  • Additional Classes not represented in lab:
    • Cubozoa: box jellies and sea wasps.
    • Staurozoa: stalked jellyfish.
    • Myxozoa: spore-producing parasites
Discharged Nematocyst of Hydra sp.

Class Hydrozoa Hydroids


  • Alternation of generations: polypoid generation usually dominant
  • Medusae usually small, transparent
  • A few (fire corals) produce coral-like calcareous skeleton
  • Polyps usually colonial, with interconnected coelenterons
  • No cells in mesoglea
  • Nematocysts only in epidermis (not gastrodermis)
  • Marine and fresh waters
Green Hydra
Green Hydra, Chlorohydra viridissima (Order Anthomedusae)
Hydra section
Hydra sp., stained longitudinal section, 100x (Order Anthomedusae)
See also labeled photo.
Hydra Reproduction
Hydra sp., stained whole mounts showing sexual and asexual reproduction, 10x
See also labeled photo.
Pennaria Hydroid Colony
Pennaria sp. hydroid colony, stained whole mount (Order Anthomedusae)
Polyorchis Jelly
Bell Jelly, Polyorchis penicillatus (Order Anthomedusae)
Blade Fire Coral
Blade Fire Coral, Millepora complanata, Belize (Order Anthomedusae)
Hydroid Colony
Eudendrium sp. hydroid colony (Order Anthomedusae)
By Wind Sailor
By-The-Wind-Sailor, Velella velella, beached, CA (Order Anthomedusae)
Angled Hydromedusa
Angled Hydromedusa, Gonionemus sp. (Order Limnomedusae)
Obelia Colony
Obelia sp. colony, stained whole mount, 40x (Order Leptomedusae)
See also labeled photo.
Obelia Medusa
Obelia sp. medusa, stained whole mount, 100x (Order Leptomedusae)
Clytia Polyps
Ostrich-plume Hydroid colony, Aglaophenia sp. (Order Leptomedusae); La Jolla, CA
Clytia Polyps
Clytia sp. hydroid colony (Order Leptomedusae); preserved specimen
Umbrella Jelly
Umbrella Jellyfish, Eutonina indicans medusa (Order Leptomedusae)
Portuguese Man-O-War, Physalia physalis, in tidepool, Kenya (Order Siphonophora)

Class Anthozoa Anemones & Corals


  • Lack medusoid stage
  • Polyps solitary or colonial
  • Coelenteron (gastrovascular cavity) subdivided by longitudinal folds of tissue
  • Tentacles in multiples of 6 (Hexacorallia) or 8 (Octocorallia)
  • Marine only
Sea anemone dissection (Metridium sp.)
Check answers on labeled photo.

Subclass Hexacorallia (Zoantharia)

Order Actiniaria

  • Anemones
  • Polyps large, usually with distinct stalk
  • Numerous tentacles
  • Lack calcified skeleton
  • Solitary or in aggregations but not truly colonial
Giant Plumose Anemone
Giant Plumose Anemone, Metridium farcimen
Rose Anemone
White-spotted Rose Anemone, Urticina lofotensis
Giant Green Anemone
Giant Green Anemone, Anthopleura xanthogrammica, CA

Order Scleractinia

  • Stony corals
  • Secrete calcareous skeleton
  • Most colonial with small polyps, though some solitary
  • But some solitary with large polyps
Stony Corals
Boulder coral, Montastrea annularis (foreground) and Brain corals, Diploria sp., Belize
Obelia Colony
Yellow pencil coral, Madracis agaricites, Belize
Elkhorn Coral
Elkhorn coral, Acropora palmata, reef, Belize
Brain coral
Brain Coral, Platygyra sp.
Pineapple coral
Moon Coral, Favites sp.
unid Stony Coral
Cauliflower Coral, probably Stylophora or Pocillopora sp.

Other Orders

  • Disk anemones, button polyps, black corals, tube anemones and others
Button Polyps
Button Polyps, Palythoa sp. (Order Zoantharia)
Mushroom Anemones
Mushroom Anemones, Discosoma sp. (Order Corallimorpharia)
Tube Anemone
Tube Anemone, Pachycerianthis sp., (Order Ceriantharia; but often placed in its own subclass)
Black Coral
Yellow-polyp Black Coral, Antipathes galapagensis (Order Antipatharia)

Extinct Fossil Corals

  • Ordovician to Permian (488-251 mya)
  • Rugose corals were solitary or colonial
  • Tabulate corals were always colonial
Sun Coral fossil, Pachyphyllum nevadens; Tonto N.F., AZ, Devonian Period (Order Rugosa)
Horn Corals
Horn Coral fossils; Tonto N.F., AZ, Devonian Period (Order Rugosa)
Coral fossil, Hexagonaria sp., polished; Morocco, Devonian Period (Order Rugosa)
Tabulate Coral
Tabulate Coral fossil; KY, Ordovician Period (Order Tabulata)
Tabulate Coral
Thamnopora sp. coral branches; Tonto N.F., AZ, Devonian Period (Order Tabulata)

Subclass Octocorallia (Alcyonaria)

Order Alcyonacea

  • Soft corals
  • Lack rigid skeleton
  • Support provided by calcareous spicules
  • Colonies often tree-like or resemble anemones
Leather Coral
Umbrella Leather Coral, Sarcophyton trochelisphoram
Kenya Tree Coral
Kenya Tree Coral, Capnella imbricata
Xenia Coral
Xenia Coral, Xenia sp.

Order Gorgonacea

  • Sea whips and sea fans
  • Small polyps form branching colonies
  • Skeleton usually proteinaceous
  • Sometimes included within the Alcynonacea
Sea Fan
Sea Fan, Gorgonia sp., Belize
Orange Gorgonian
Orange Gorgonian, Adelogorgia phyllosclera?
Red Gorgonian
Red Gorgonian, Lophogorgia chilensis

Other Orders

  • Organ pipe corals & sea pens
Organ Pipe Coral
Organ Pipe Coral, Tubipora musica, skeleton (Order Stolonifera)
Elkhorn Coral
Fleshy Sea Pens, Ptilosarcus sp. (Order Pennatulacea)

Class Scyphozoa Jellyfish


  • Medusoid stage predominates; polyps inconspicuous
  • Thick gelatin-like mesoglea
  • Often pigmented
Moon Jelly
Moon Jelly, Aurelia labiata, adult medusa (Order Semaeostomae)
See also labeled photo.
Moon Jelly Polyps
Moon Jelly, Aurelia labiata, polyps
Moon Jelly Life Cycle
Moon Jelly, Aurelia sp., life cycle stages; stained whole mounts
Northern Sea Nettle
Northern Sea Nettle, Chrysaora melanaster (Order Semaeostomae)
Pacific Sea Nettle
Pacific Sea Nettle, Chrysaora fuscescens (Order Semaeostomae)
Lions Mane
Lion's Mane Jelly, Cyanea capillata (Order Semaeostomae)
Spotted Jellyfish
Spotted Jellyfish, Mastigias papua (Order Rhizostomae)
Upside Down Jellyfish
Upside Down Jellyfish, Cassiopea andromeda (Order Rhizostomae). This species relies on endosymbiotic algae.
Conularia fossil
Conularids, such as these Conularia sp., are an extinct group thought to be related to the Sycphozoa (due to their four-fold symmetry). They probably attached at the narrow end, with tentacles at the upper end (Order Conulariida).
This page last updated 21 August 2016 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.