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Phylum Ctenophora
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A brief description of the main characteristics and features of phylum Ctenophora.
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CTENOPHORA -GENERAL CHARACTERSINTRODUCTION
Ctenophora are marine forms having biradial symmetry, gastrovascular cavity and
gelatinous mesenchymal mesoglea with associated muscle cells. The principal
distinctive characteristic is the radially arranged rows of cilia or ctenes. Tentacles
are not arranged in whorls around the mouth. They have an aboral sense organ and
special adhesive cells or colloblasts. Cleavage is determinate and ontogeny
culminates in a cydippid larva with virtually no planula.
CTENOPHORA Vs CNIDARIA
Like jellyfish the main axis of Cnidarian body is oral and aboral and the mesoglea
is thick and buoyant. But Ctenophores differ from jellyfish in two striking ways:
1. Ctenophores use cilia instead of muscles for locomotion.
2. Ctenophores lack cnidae.
Because of these differences and some unique attributes of their own, Ctenophores
are classified separate from Cnidarians. Historically Ctenophores and Cnidarians
were combined in the now defunct taxon Coelenterata.
GENERAL BIOLOGY
1. Ctenophores have biradial symmetry about the two imaginary planes which
divide the body into mirror image halves:
a. The tentacular plane- passes along the oral aboral axis and through
both tentacle sheaths.
b. The pharyngeal plane- this plane also passes along the oral-aboral axis,
but at a right angle to the tentacular plane
NOTE: This is in contrast to the Cnidarians which have many radial
planes of symmetry whereas Ctenophores have only two.
, 2. Between the two poles are eight rows of extraordinary cilia that can move
the large body at speeds up to 5Smm/sec. The cilia in each row are
exceptionally long (up to 2mm) and grouped together in paddles known as
ctenes. Each ctene is composed of several thousand cilia. The paddle like
ctenes beat with effective strokes directed aborally. Light diffracted by the
moving ctenes produces a shimmering spectrum of colo. NOTE; In addition
to the ctenes, Ctenophores use other highly modified cilia as specialized
devises such as pacemakers (balancers) to set the beat rate of the ctenes and
even as teeth (Example: macro cilia in Beroe).
3 Two long tentacles, one on each side of the body originate on the aboral
hemisphere and bear a lateral row of thread like filaments called tentilla.
Each highly extensible and contractile tentacle emerges from the bottom of a
deep ciliated pouch, the tentacle sheath.
4. The body is covered by epidermis containing many gland cells, sensory cells
and pigment granules. Internal to the mouth, the coelenteron consists of a
large compressed pharynx that opens into a short canal from which
additional canals arise.
5. Much of the body volume is occupied by the skeletal mesoglea, an elastic
gel that contains collagen and cells of various types. Mesoglea contains
amoebocytes, connective tissue fibers, muscle fibers and nerve fibers to the
muscles.
6. The epidermis of each tentacle and tentillium has numerous specialized cells
called collocytes, that are unique to Ctenophores. The collocytes consist of a
swollen apical cap borne on a stalk. The cap bears numerous vesicles that
contain the adhesive substance. Collocytes release a sticky substance and
adhere to the prey.
RESPIRATION
There is no special respiratory structure in Ctenophores. Ctenophores resemble
Cnidarians in relying on water flow through the body cavity for respiration.
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