Zoology 250 Lecture 9
PHYLUM CNIDARIA (II): Diversity
(Cnidaria study images)
- 1) Life cycle: asexual reproduction & coloniality are widespread; most forms show alternation of generations (polyp <-> medusa); external fertilization; radial cleavage; both blastula & gastrula stages
- a) a distinctive ciliated planula larva forms after gastrulation
- b) the planula larva may be the ancestral form of the Bilateria
- 2) Comparisons among classes:
Characteristics |
Cl. Hydrozoa |
Cl. Scyphozoa |
Cl. Anthozoa |
Both polyp & medusa? |
yes |
yes (most) |
polyp only |
Polyp phase: |
-- mouth |
simple |
simple |
tubular pharynx |
-- gastrovasc. septae |
no |
yes |
yes |
-- gastric filaments |
no |
yes |
yes |
Medusa phase: |
-- velum |
present |
absent |
- - - |
-- bell margin |
simple |
notched (rhopalia) |
- - - |
-- manubrium |
simple |
complex |
- - - |
-- gastrovasc. canals |
unbranched |
branched |
- - - |
-- gastric filaments |
no |
yes |
- - - |
Gonads |
ectodermal |
endodermal |
endodermal |
- 3) Cl. HYDROZOA (hydroids & relatives) exhibit a bewildering array of life-cycles; some emphasize the polyp and others emphasize the medusa stage:
- a) Hydroids- solitary or colonial polyp form predominates
- polyps thecate (e.g., Obelia) or naked (e.g., Hydra); distinguishes two orders
- division of labor among polyps in colonial forms
- most still possess a medusa stage for sexual reproduction
- b) Trachyline medusae- the polyp stage is reduced or absent
- c) Hydrocorals- polymorphic polyps form colonies with massive carbonate skeletons; medusa stage reduced
- d) Or. Siphonophora- swimming or floating polymorphic colonies; may have multiple medusoid & polypoid forms in one colony!
- 4) Cl. SCYPHOZOA (jellyfish), the medusa form predominates
- a) new medusae are produced from a polyp by strobilation
- b) some polyp stages possess septae in the gastrovascular cavity
- 5) Cl. ANTHOZOA (anemones, corals), the medusa form is absent
- a) Octocorals- diverse colonial forms (soft corals, gorgonians & sea pens); typically have 8 pinnate tentacles & 8 septae
- the tanned collagen skeleton of gorgonians is an endoskeleton
- b) Hexacorals- a heterogeneous group (solitary anemones & stony corals); more than 8 tentacles & septae (typically multiples of 12)
- the massive carbonate skeleton of corals is an exoskeleton
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Copyright © 2000 by A. Richard Palmer. All rights reserved.
(revised Mar. 28, 2000)