1) Well defined phylum of ~10,000, mostly marine, predatory species divided into three main classes (Hydrozoa, Scyphozoa, Anthozoa)
2) Two body forms predominate: polyp (benthic) and medusae (pelagic), alternation of generations is widespread (not haplo-diploid as in plants)
3) The basic body plan of all cnidarians is similar:
a) radially symmetrical with three well-defined layers to body wall
b) the gut has one opening (mouth only, no anus)
c) tentacles (hollow or solid) surround the end bearing the mouth; they usually capture prey and transport food to the mouth
d) the gastric cavity has associated canals (=gastrovascular cavity)
4) Cnidarians exhibit a true, three-layered, tissue level of organization (groups of differentiated cells working together for a single function)
a) epidermis (outer) and gastrodermis (inner) are true tissues
b) the middle layer (mesoglea) mostly non-cellular (simple basal lamina or massive & gelatinous); not true tissue, so Cnidaria are diploblastic
5) This diverse phylum is united by a unique and complex cell type:
a) a cnidocyte (drived from endodermal interstitial cells) is a cell that bears two important organelles:
nematocyst (capsule with an inverted coiled tube and operculum)
cnidocil (sensory cilium= "hair trigger")
6) Nematocysts come in >30 forms (grappling, penetrating, sticky); one to several forms may occur in a single individual; they aid in feeding (mainly), and also in defense and aggression
7) Only a few other cell types exist; most are in epidermis and gastrodermis:
a) epithilio-muscular (cells have basal myofibrils; dual function: skin and muscle; myofibrils are longitudinal in epidermis, circular in gastrodermis); Scyphozoa & Anthozoa may also have true, striated-type myocytes (muscle cells) that move to mesoglea
b) interstitial (totipotent cells; produce gametes and cnidocytes)
c) receptor (often have sensory cilium)
d) nerve (form loosely organized net); no myofibrils
e) gland (mainly in endodermis); myofibrils
f) amoeboid (mobile cells in mesoglea; from epidermis of Scyphozoa)
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