BIO 469 Lecture Notes - Lecture 8: Symmetry In Biology, Gastrovascular Cavity, Cnidocyte
13 Jun 2018
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Cnidaria
● 11,000 Species
○ Mostly marine, some freshwater
● Size Range: Microscopic to individual jellies 2m wide and with tentacles 25m long
● Motion: Sessile; Polyps and/or planktonic; Medusae
● Biphasic life cycle
● Have planula larvae (free swimming); asexual reproduction is common
● Carnivores; suspension feeding or they harbor symbiotic intracellular algae;
zooxanthellae (Some are parasitic)
Characteristics of Phylum
● “Diploblastic”; ectoderm and endoderm separated by a ectodermal derived cellular
mesoglea or partly cellular mesenchyme
● Primarily radial symmetry; the primary body axis is oral-aboral (anteroposterior)
● Exhibit alternation of asexual polyploid ands sexual medusoid generations but there
are many variations on this basic theme (biphasic life cycle)
● Musculature formed largely of myoepithelial cells, derived from ectoderm and
endoderm
● The endodermal derived gastrovascular cavity (coelenteron) is the only “body cavity”
● Possess unique stringing or adhesive structures called cnidae; each cnida resides in
and is produced by one cell, a cnidocyte. The most common cnidae are called
Nematocysts.
● No head, centralized nervous system, or discrete gas exchange, excretory, or circulatory
structures
● Nervous system is a simple nerve net(s), composed of naked and largely non-polar
neurons; presence of sensory structures
Body Symmetry
● An organism with radial symmetry has one main body axis around which its body parts
are arranged. A perfectly radially symmetrical animal can be divided into similar halves
by any plain that contains the main axis
○
● An animal with bilateral symmetry can be divided into mirror images (left and right) by
single plane that passes through the dorsoventral midline of its body from the anterior to
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the posterior end. Dorsal/ventral. Bilateral symmetry is usually correlated with
cephalization.
Biphasic Life History - Medusozoa
● The polyp is sessile and reproduces asexually by budding
● The medusa is a free swimming stage and serves for sexual reproduction and dispersal
Tissue Layers:
● 2 Tissue layers
○ Outer layer; epidermis ( protective)
○ Inner layer: gastrodermis (lines gut cavity)
■ Between tissue layers is a non-cellular, non-living, jelly-like material called
mesoglea
Cnidae- unique to Cnidaria
● Single cell that when loaded have a structure that is barbed with a filament.
○ Cnidocil flips inside out
● The barbs are for capturing
Support Structures
● Axial skeletons (organic skeleton, no CaCO3) of protein-mucopolysaccharide complexes:
some gorgonians, sea pens, black corals
● Calcareous sclerites(collagen); Some gorgonians
● Massive calcareous skeletons: anthozoan corals (Scleractinia)
● Exoskeletons (perisac) of chitin and proteins in thecate hydroids and some scyphozoans
Sensory Structures
● Rhopalia of scyphomedusae
○ Rotation of the jelly
○ Photoreceptor for light sensing
● Hydrozoans have a lens, and nerve cells deliver impulses towards lens
● Cubozoans have developed a true eye; cornea, retina, and lens, Can sense where light
is coming from
○ Same structure arose 3 times each time with a similar structure
● Diffuse mesh of nerve cells that take part in simple reflex pathways
● Nerve cells interact with sensory and contractile cells
Movement
● Myoepithelial cells provide contraction for propulsion
● Retractor muscles and circular muscles work with the GVC to function as a hydrostatic
skeleton
● Cilia propel pelagic larvae
● Filtering way more water than other animals of their size, in calm waters and open ocean
the filtering works best
Feeding
● Carnivorous: use cnidae, mucus nets or direct capture
● Extra/intracellular digestion
● Some with algal symbionts
● Some parasites
Reproduction
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Document Summary
Size range: microscopic to individual jellies 2m wide and with tentacles 25m long. Have planula larvae (free swimming); asexual reproduction is common. Carnivores; suspension feeding or they harbor symbiotic intracellular algae; zooxanthellae (some are parasitic) Diploblastic ; ectoderm and endoderm separated by a ectodermal derived cellular mesoglea or partly cellular mesenchyme. Primarily radial symmetry; the primary body axis is oral-aboral (anteroposterior) Exhibit alternation of asexual polyploid ands sexual medusoid generations but there are many variations on this basic theme (biphasic life cycle) Musculature formed largely of myoepithelial cells, derived from ectoderm and endoderm. The endodermal derived gastrovascular cavity (coelenteron) is the only body cavity . Possess unique stringing or adhesive structures called cnidae; each cnida resides in and is produced by one cell, a cnidocyte. No head, centralized nervous system, or discrete gas exchange, excretory, or circulatory structures. Nervous system is a simple nerve net(s), composed of naked and largely non-polar neurons; presence of sensory structures.
