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GENERAL CHARACTERS OF CEPHALOCHORDATES

B.SUJATHA,

GOVT. DEGREE COLLEGE,

RAJAMPET

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GENERAL CHARACTERS OF CEPHALOCHORDATES

  • Marine, widely distributed in shallow waters.
  • Mostly sedentary and buried with only anterior body end, projecting above bottom sand.
  • Body small, 5 to 8 cm long, slender, fish-like, segmented and transparent.
  • Body has trunk and tail. Head lacking.
  • Paired appendages lacking. Median fins present.

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GENERAL CHARACTERS OF CEPHALOCHORDATES

  • Exoskeleton absent. Epidermis single-layered.
  • Muscles dorso-lateral, segmented into myotomes.
  • Coelom enterocoelous, reduced in the pharyngeal region by development of atrial cavity.
  • Notochord rod-like, persistent, extending from rostrum to tail, hence, the name Cephalochordata.
  • Digestive tract complete. Pharynx large, perforated by numerous persistent gill-slits opening into atrium.
  • Respiration through general body surface. Special organs for respiration absent.

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GENERAL CHARACTERS OF CEPHALOCHORDATES

  • Circulatory system well developed, closed without heart and respiratory pigment. Hepatic portal system developed.
  • Excretion by protonephridia with solenocytes.
  • Nerve cord dorsal, tubular without ganglia and brain. Dorsal and ventral nerve roots separate.
  • Sexes separate. About 25 pairs of gonads. Gonoducts lacking. No asexual reproduction.
  • Fertilisation external in sea water.
  • Development indirect, including a free-swimming larva.
  • Cephalochordata has about 30 species of the genus Branchiostoma and all put in the class Leptocardii.

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AFFINITIES OF BRANCHIOSTOMA:�

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  • Branchiostoma show affinities with non-chordate as well as chordates.
  • Cephalochordates have been regarded to be phylogenetically related to several non-chordate groups at one time or other.
  • But these can be overlooked in favour of the more chordate-like characteristics of cephalochordates.

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CHORDATE AFFINITIES:

  • Cephalochordata (Branchiostoma) exhibits the three basic chordate features such as notochord, dorsal tubular nerve cord and pharyngeal gill-slits in the most typical manner and there is no doubt about its chordate nature.
  • However, it exhibits relationships with all the major groups of phylum chordata, and its real status in the phylum remains uncertain.

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AFFINITIES WITH HEMICHORDATA:�

Hemichordata and Cephalochordata resemble in having similar:

  • Pharyngeal apparatus with numerous gill-slits and gill-bars,
  • Filter feeding mechanism.
  • Respiratory mechanism.
  • Enterocoelous coelom.
  • Numerous gonads without gonoducts.

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AFFINITIES WITH UROCHORDATA:

  • The primitive ciliary feeding and respiratory mechanisms,
  • A large pharynx having many gill-clefts (stigmata), an endostyle which produces mucus, peripharyngeal bands and an epipharyngeal groove (dorsal lamina),
  • An ectoderm-lined atrial cavity opening to the exterior by an atriopore or its homologue, the atrial siphon,
  • The mantle of Herdmania is equivalent to the atrial folds of Branchiostoma. Besides the homologies of adult anatomy, there are embryonic similarities.

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OBJECTIONS:

  • Body unsegmented and covered by a cellulose test,
  • Without hollow nerve cord and notochord,
  • With liver,
  • With a heart covered by peritoneum,
  • Absence of nephridia,
  • Sexes united and gonads hermaphrodite, and
  • Larva undergoes retrogressive metamorphosis.

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AFFINITIES WITH CYCLOSTOMATA:

  • Elongated, slender fish-like body,
  • Continuous dorsal median fin,
  • Mouth surrounded by an oral hood,
  • Presence of a velum,
  • Pharynx with an endostyle and gill-slits, and
  • Presence of dorsal hollow nerve cord and notochord.

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AFFINITIES WITH OTHER VERTEBRATES:

  • Post-anal tail,
  • Metameric myotomes,
  • Coelom lined by mesodermal epithelium,
  • Mid-gut diverticulum comparable with liver,
  • Well-formed hepatic portal system, and
  • With forward flow of blood in ventral and backward flow of blood in dorsal longitudinal blood vessels.

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UROCHORDATA 

  • They are marine, mostly sessile, filter–feeders.
  • Body is enclosed in a leathery test or tunic sheath, composed of tunicin (cellulose) so called tunicates.
  • The notochord occurs only in the tail of larva and disappears in the adults called retrogressive metamorphosis.
  • The nerve chord (neural tube) is present in the larva, but is replaced by a single dorsal ganglion in the adult.
  • The gill slits or stigma are numerous, persist in the adults and open into the atrium, instead of opening to the exterior. There are no true gills but are called branchial basket.
  • Circulatory system is of open type. Blood consists of Venadocytes.
  • Excretory system is lacking.
  • Asexual reproduction occurs by budding.

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