Pete Oxford / Bladed Fire Coral (Millepora Complanata) causes sting on contact, Belize Barrier Reef, Belize

Hiroya Minakuchi / Flower Hat Jelly (Olindias formosa), native to Brazil, Argentina and Japan

Ingo Arndt / Hydromedusae (Calycopsis borchgrewinki) with extended tentacles, Weddell Sea, Antarctica

David Shale / Siphosome of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea), North Pacific Ocean

Flip Nicklin / By-the-wind Sailor (Velella velella) jellyfish, on water surface

Juan Manuel Borrero / Illustration of formation of jellyfish Hydrozoa. Pencil and watercolor painting.

Juan Manuel Borrero / Illustration of individuals of Velella (Velella velella) siphonophore Colony. Velella is a small hydrozoan, 1 to 5 cm which floats on the sea. Pencil and watercolor painting.

Norbert Wu / Antarctic Sun Starfish (Labidiaster annulatus), Hydrozoan (Candelabrum penola), and Sponge (Mycale acerata) below kelp forest, Palmer Station, Antarctic Peninsula, Antarctica

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea), Prince William Sound, Alaska

Christian Ziegler / Fire Coral (Millepora alcicornis) growing directly beneath Red Mangroves (Rhizophora mangle), Bastimentos Marine National Park, Bocas del Toro, Panama

Christian Ziegler / Fire Coral (Millepora alcicornis) growing directly beneath Red Mangroves (Rhizophora mangle), Bastimentos Marine National Park, Bocas del Toro, Panama

Christian Ziegler / Fire Coral (Millepora alcicornis) growing directly beneath Red Mangroves (Rhizophora mangle), Bastimentos Marine National Park, Bocas del Toro, Panama

Nature Production / Hydrozoan jellyfish (Cladonema japonica / uchidai) Japan

Pete Oxford / Fire Coral (Millepora alcicornis) with fine hairs that contain nematocysts which sting on contact, Belize Barrier Reef, Belize

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea) in aquarium, Japan

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea) in aquarium, Japan

Konrad Wothe / Hydra (Hydra vulgaris), Germany

Fred Bavendam / Solitary Hydroid (Corymorpha pendula) filter feeding, Isles of Shoals, New Hampshire

Norbert Wu / Pink Helmet (Aglantha digitale) jellyfish, Arctic Ocean

Norbert Wu / Pink Helmet (Aglantha digitale) jellyfish, Arctic Ocean

Hiroya Minakuchi / Flower Hat Jelly (Olindias formosa), native to Brazil, Argentina and Japan

Norbert Wu / Jellyfish (Aequorea sp) swimming, Vancouver Island, British Columbia, Canada

Scott Leslie / Constricted Jellyfish (Catablema vesicarium), Nova Scotia, Canada

Scott Leslie / Hydromedusae with hitch-hiking larvae, Nova Scotia, Canada

Scott Leslie / Hydromedusae with hitch-hiking larvae, Nova Scotia, Canada

Scott Leslie / Acadian Hermit Crab (Pagurus acadianus) with Snail Fur Hydroid (Hydractinia sp) growing on its shell, Nova Scotia, Canada

Pete Oxford / By-the-wind Sailor (Velella velella) group and other animals washed up on beach, North Stradbroke Island, Australia

Albert Lleal / Hydra (Hydra sp), native world wide

Hiroya Minakuchi / Jellyfish (Spirocodon saltator) spreading tentacles, aquarium, Japan

Hiroya Minakuchi / Jellyfish (Tima formosa) spreading tentacles, aquarium, Japan

Chris Newbert / Harlequin Ghost Pipefish (Solenostomus paradoxus) male camouflaged against Hydroids (Cnidoscyphus sp) 50 feet deep, Solomon Islands

Ingo Arndt / Hydromedusae (Calycopsis borchgrewinki) with extended tentacles, Weddell Sea, Antarctica

Ingo Arndt / Jellyfish (Leuckartiara sp), Weddell Sea, Antarctica

Ingo Arndt / Jellyfish (Leuckartiara sp), Weddell Sea, Antarctica

Ingo Arndt / Hydromedusae (Calycopsis borchgrewinki) with reflection on water surface, Weddell Sea, Antarctica

Kim Taylor / Colony of marine hydroid obelia and red algae

David Shale / Cormidium of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / Palpon + gasterozooids of {Forskalia asymmetrica} Atlantic, siphonophore hydrozoan

David Shale / Siphonophore hydrozoan cnidarian {Nanomia cara} Atlantic, close up of pneumatophore

David Shale / Siphonophore hydrozoan cnidarian {Nanomia cara} close up of nectophores, gasterozooid and palpons

David Shale / Deep sea Siphonophore, hydrozoan cnidarian {Bargmannia sp} 2503 ft, Gulf of Maine

David Shale / Cormidium of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / Palpon + gasterozooids of {Forskalia asymmetrica} Atlantic, siphonophore hydrozoan

David Shale / Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / {Halitrephes maasi} pelagic hydrozoan medusa from Gulf of Maine, Atlantic.

David Shale / Palpon + gasterozooids of {Forskalia asymmetrica} Atlantic, siphonophore hydrozoan

David Shale / Cormidium of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / Deep sea Siphonophore, hydrozoan cnidarian {Bargmannia sp} 2503 ft, Gulf of Maine

David Shale / Hydrozoan cnidarian {Stephanomia amphitridis} deepsea, Atlanticat 2715 ft, gasterozooid and palpons

David Shale / Palpon + gasterozooids of {Forskalia asymmetrica} siphonophore hydrozoan, Gulf of Maine Atlantic, N W Atlantic, depth 600-800m

David Shale / Cormidium of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / {Halitrephes maasi} pelagic hydrozoan medusa from Gulf of Maine, Atlantic.

David Shale / Palpon + gasterozooids of {Forskalia asymmetrica} Atlantic, siphonophore hydrozoan

David Shale / Siphonophore hydrozoan cnidarian {Nanomia cara} Atlantic.

David Shale / Siphonophore hydrozoan cnidarian {Nanomia cara} close up of nectophores, gasterozooid and palpons

David Shale / Cormidium of Hydrozoan cnidarian {Stephanomia amphitridis} deep sea, Atlantic.

David Shale / {Halitrephes maasi} pelagic hydrozoan medusa from Gulf of Maine, Atlantic.

David Shale / Deep sea Siphonophore, hydrozoan cnidarian {Bargmannia sp} 2503 ft, Gulf of Maine

David Shale / Siphonophore hydrozoan cnidarian {Nanomia cara} Atlantic - nectophores and cormidium

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea), North Pacific Ocean

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea) drifting, underwater, North Pacific Ocean

Hiroya Minakuchi / Portuguese Man of War (Physalia physalis) floating on ocean surface, Portugal

Norbert Wu / Fire Coral (Millepora alcicornis) a hydrocoral, stings fiercely on contact, Caribbean

Norbert Wu / Hula Skirt Siphonophore (Physophora hydrostatica) colonial animal, tentacles sting powerfully, grows to 41 cm, inhabits the Arctic Ocean and Pacific Ocean

Norbert Wu / Fire Coral (Millepora alcicornis) stings fiercely, takes many shapes- note stinging bristles, Caribbean

Norbert Wu / Fire Coral (Millepora alcicornis) stings fiercely, takes many shapes- note stinging bristles, Caribbean

Norbert Wu / Constricted Jellyfish (Catablema vesicarium) tiny jellyfish, Arctic

Norbert Wu / Narcomedusae (Aegina sp) with numerous parasitic amphipods within bell, Arctic

Norbert Wu / By-the-wind Sailor (Velella velella) siphonophore allows the jellyfish to float and the sail catches the wind, California

Norbert Wu / Portuguese Man of War (Physalia physalis) a siphonophore colony split into work groups, poisonous, Belize

Norbert Wu / Portuguese Man of War (Physalia physalis) a siphonophore colony split into work groups, poisonous, Belize

Norbert Wu / Hyperiid (Hyperia medusarum) rides on long muscular gut of Jellyfish (Sarsia sp) and steals food, Arctic

Norbert Wu / Pink Helmet (Aglantha digitale) jellyfish underwater, Arctic

Norbert Wu / Hula Skirt Siphonophore (Physophora hydrostatica) colonial animal, tentacles sting powerfully, grows to 41 cm, inhabits the Arctic Ocean and Pacific Ocean

Fred Bavendam / Elegant Hydrocoral (Stylaster elegans) growing on coral reef, Manado, North Sulawesi, Indonesia

Hiroya Minakuchi / Crystal Jellyfish (Aequorea aequorea), Prince William Sound, Alaska

Christian Ziegler / Bladed Fire Coral (Millepora complanata) on reef, Bastimentos Marine National Park, Bocas del Toro, Panama

Christian Ziegler / Fire Coral (Millepora alcicornis) growing directly beneath Red Mangroves (Rhizophora mangle), Bastimentos Marine National Park, Bocas del Toro, Panama