Phylum Hemichordata - NEET-Biology

 Phylum Hemichordata

Fig: Acorn worm (Hemichordate)

• Hemichordate = half chordate.

• It shares some (but not all) of the typical chordate characteristics.

• These are not classified as true chordates, although they are closely related.

• Some DNA-based studies evolution suggest that hemichordates are actually closer to echinoderms than to true chordates.

• It was earlier considered as a sub-phylum under the phylum Chordata. 

• But, now it is placed under a separate phylum under non-chordates.

Important characteristics:

• Exclusively marine.

• Worm-like animals with the organ-system level of organization.

• Bilaterally symmetrical, triploblastic & coelomate animals.

• The body is cylindrical & is composed of an anterior proboscis, a collar & a long trunk.

• Notochord present which allows & endodermal in origin (termed stomochord).

• Notochord occurs only in the anterior end of the body. It is called the buccal diverticulum.

• Open circulatory system.

• Perform sexual reproduction.

• Sexes are separate.

• Fertilization is external.

• Development is indirect.

• Example: Balanglossus (Tongue worm), Ptychodera & Saccoglossus.

Fig: Balanoglossus (Tongue worm)