GENERAL CHARACTERISTICS OF HEMICHORDATA
Greek hemi, means “half,” and Latin chorda, means “string,”
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There are only about 90 known species.
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They are exclusively marine and benthic.
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Triploblastic animals i.e., body is derived from 3
embryonic germ layers (ectoderm, mesoderm and
endoderm).
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Body is bilaterally symmetrical.
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They are the deuterostomes. Body is vermiform and is
differentiated into Proboscis (=prososome), collar
(=mesosome) and trunk (=metasome). Body wall is
covered by a single layered epidermis.
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Body segmentation is not seen in these animals.
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Oragan-system grade of body oraganisation is seen.
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Coelom is enterocoelous, divisible into proctocoel,
mesocoel and metacoel.
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Proboscis acts as locomotory and food capturing
organ.
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Ciliated pharyngeal gill-slits are present
(Respiratory organ).
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Notochord is absent. Buccal diverticulum present in
the proboscis was earlier considered as notochord.
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In Enteropneusta, body wall comprises of both
circular and longitudinal muscles ,but in
Pterobranchia, only longitudinal muscles are
present in the body wall.
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Tornaria larva is the larval stage.
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They are the suspension /filter feeder.
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Digestive system is complete type, with a buccal
diverticulum (stomochord) in the proboscis.
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Excretion by single glomerulus situated in the
proboscis.
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Circulatory system is open type.
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Nerve cord is occasionally hollow, soft, mesodermal,
and probably is homologous with chordates nerve
cord.
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Sensory cell of the epidermis acts as sense organs.
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Mostly sexual reproduction occurs, but asexual
reproduction is also seen.
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Generally, development is indirect, through Tornaria
larval stage.
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Fertilisation is exernal.
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Mostly dioecious forms are found.
