Biology Sikshya

     Greek hemi, means “half,” and Latin chorda, means “string,”

• There are only about 90 known species.

• They are exclusively marine and benthic.

• Triploblastic animals i.e., body is derived from 3

       embryonic germ layers (ectoderm, mesoderm and
       endoderm).

• Body is bilaterally symmetrical.

• 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.

• Body segmentation is not seen in these animals.

• Oragan-system grade of body oraganisation is seen.

• Coelom is enterocoelous, divisible into proctocoel,

       mesocoel and metacoel.

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• Proboscis acts as locomotory and food capturing

       organ.

• Ciliated pharyngeal gill-slits are present

       (Respiratory organ).

• Notochord is absent. Buccal diverticulum present in

       the proboscis was earlier considered as notochord.

• In Enteropneusta, body wall comprises of both

       circular and longitudinal muscles ,but in
       Pterobranchia, only longitudinal muscles are
       present in the body wall.

• Tornaria larva is the larval stage.

• They are the suspension /filter feeder.

• Digestive system is complete type, with a buccal

       diverticulum (stomochord) in the proboscis.

• Excretion by single glomerulus situated in the

       proboscis.

• Circulatory system is open type.

• Nerve cord is occasionally hollow, soft, mesodermal,

       and probably is homologous with chordates nerve
       cord.

• Sensory cell of the epidermis acts as sense organs.

• Mostly sexual reproduction occurs, but asexual

       reproduction is also seen.

• Generally, development is indirect, through Tornaria

       larval stage.

• Fertilisation is exernal.

• Mostly dioecious forms are found.