Gill-slits of Balanoglossus are numerous and dorsal in position, whereas they are 5 to 7 and lateral in higher chordates.In Balanoglossus, the dorsal tubular nerve cord is confined to the collar region only. The nervous system is distinctly of the invertebrate type being intra-epidermal in position and having a ventral nerve cord and a circumenteric nerve ring which area absent in chordates.Instead of being called notochord, it is now termed the Some prefer to name it as buccal diverticulum. It does not originate from the roof of larval archenteron but as a forward hollow projection of the foregut. Instead of being solid and made of vacuolated cells, it is hollow and lined by epithelial cells. It is ventral to the main (dorsal) blood vessel and not covered by sheaths. Unlike that of the chordates, the so-called ‘notochord’ is very short, confined to proboscis and without any supporting function. A true notochord does not occur in hemichordates.However, the hemichordates are no longer included under chordates because they do not possess chordate characters in a typical condition.
Besides similarities in the structure and function of the branchial apparatus, the hemichordates also show similarity with cephalochordates in the arrangement of coelomic sacs and in development.ĭue to these similarities Hemichordata had been considered as a subphylum of the phylum Chordata till recently, representing its lowest group, and probably having a common ancestry.
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