Invertebrate Zoology
Lecture 7: Phylum Cnidaria,
Part 2
Class Hydrozoa
Siphonophore Photo:
K. Raskoff / Monterey Peninsula College
Class Hydrozoa
 Order Hydroidea: the hydroids
Polyp form often colonial, branched (Obelia)
Includes feeding and reproductive polyps
Coenosarc: common gastrovascular cavity
Class Hydrozoa
 Order Hydroidea: life cycle (Obelia sp.)
Planula larvae
 Some solid, others with g.v. cavity
 Move via cilia
Class Hydrozoa
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Order Siphonophora
 Two medusa types
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Swimming
Sexual Reproduction
 Polyps: feeding,
defense, asexual
reproduction
 Predators!
 Gas-filled float
 Many variations!
Class Hydrozoa
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Order
Siphonophora
 Cormidium
 “colony within a
colony”
 Detachable
Class Hydrozoa
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Order Chondrophora
(Velella velella, by-thewind sailors):
 Free-floating polyp
 Secondary polyps (feeding,
reproduction)
 Tiny, free-living medusae
Class Hydrozoa
 Order Trachylina: life cycle
No true polyp stage
Planula develops into medusa
Class Hydrozoa

Order Hydrocorallina: hydrocoral, fire coral
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Not a “true” coral! (True coral = C. Anthozoa)
Calcareous skeleton
Some with symbiotic zooxanthellae
Polyps within chambers
Reduced medusa stage
Why called “fire” coral?
Class Scyphozoa
 Semaeostomae (most jellies)
Key predators (impact?)
 Stauromedusae
Sessile medusa or polyp?
Develop from planula larva
Class Scyphozoa
 Life cycle, Semaeostomae (Aurelia)
Key: strobilation of polyp  ephyra  medusa
Class Scyphozoa
 Pelagia
Note trailing mouth lobes
Class Scyphozoa
 Life cycle, Pelagia
No polyp stage: planula  ephyra medusa
Class Cubozoa: box jellies
Polyp  medusae via
direct metamorphosis
Chironex: deadliest
toxin known
www.zoologie-online.de/.../Cubozoa/cubozoa.html
Class Anthozoa
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Anemones
Coral
Sea pansies
Gorgonians
Etc…
Class Anthozoa
 Generalized life history: no medusae
Asexual reproduction also important
Class Anthozoa: Subclass Hexacorallia
 Order Actiniaria: solitary anemones
Class Anthozoa: Subclass Hexacorallia
 Order Corallomorpha
Anemone-like, but with clubbed tentacles
Clonal form shown; clone wars occur!
Class Anthozoa: Subclass Hexacorallia
 Order Scleractina: stony (“true”) corals
Deposit CaCO3 skeletons
Contain mutualistic zooxanthellae
Some are reef builders
Clubbed tentacles; polyps open at night…
Class Anthozoa: Subclass Hexacorallia
 Order Scleractina: stony (“true”) corals
Gastrovascular cavity connects polyps
Mutualistic zooxanthellae within gastrodermis
= acontia
Mesentery =
Class Anthozoa: Subclass Hexacorallia
 Order Scleractinia: stony (“true”) corals
Initial colony: from planula larva produced
sexually (VIDEO!)
Colony growth: asexual division
Fragmentation  “new” colony
Class Anthozoa: Subclass
Ceriantipatharia
 Order Ceriantharia: tube anemones
Double whorl of tentacles
Build sand tubes; capable of burrowing
Class Anthozoa: Subclass Octocorallia
 Features of subclass
Eight-tentacled, pinnate polyps
Eight complete mesenteries
Class Anthozoa: Subclass Octocorallia

Order Gorgonacea: sea whips, sea fans
 skeleton of gornonin (protein), and/or
calcareous spicules
 often highly branched in one plane
 Oriented perpendicular to prevailing current
Class Anthozoa: Subclass Octocorallia
 Order Alcyonacea: soft corals
 fleshy, sometimes tree-like
 pump water throughout the colony
Scubatravel.com
www.freeinfosociety.com
Class Anthozoa: Subclass Octocorallia
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Order Pennatulacea: sea pens, sea pansies
 Main body = polyp; has secondary, lateral polyps
 Gastrozooids (feeding)
 Siphonozooids (water flow)
 Calcareous spicules provide structure
© Alvaro E. Migotto
Class Anthozoa: Subclass Octocorallia
 Order Stolonifera:
organ pipe coral
 Calcareous "spicules"
provide structure
 Polyps within tubes or
on stalks