Ch 13 Cnidarians and Ctenophorans

Phylum Cnidaria

Colonial; possess cnidocytes; 700my fossil history; mostly marine; mostly sessile

Mutualism

Both individuals benefit; zooanthelles and cnidarians

Commensalism

One individual benefits, one experiences no effect

Parasitism

One individual benefits, one experiences a loss

Cnidarian characteristics

Cnidocytes; aquatic; polyps and medusae; diploblasitc; extracellular digestion; simple nerve net; no excretory or respiratory system; no coelomic cavity

Polyp

Sessile; most corals; asexual; strong phenotypic plasticity/polymorphism

Medusa

Mobile; jellyfish; bell or umbrella shaped bodies; tetramerous symmetry; statocysts and ocelli

Phenotypic plasticity  

Single genotype can have multiple phenotypes

Statocyst

Allows sensing for gravity

Ocelli

Light reception “eyes”

Cnidarian body wall

Outer endodermis, inner gastrodermis, meosglea between; no true muscle cells

Mesoglea

Covers body; thickest in stalk, thinnest in tentacles; gives support through hydrostatic pressure

Hydrostatic skeleton

Liquid pushing outward gives resistance