Sub-Phylum : Hemichordata

• Body is soft and unsegmented divided into proboscis, collar and trunk.
• Bilaterally symmetrical, triploblastic, eucoelomate.
• Digestive tract complete, respiration occurs through gills.
• Blood colourless without corpuscles.
• Sexes separate, development direct or indirect, larva tornaria.
• Restriction of ‘notochord’ to the anterior part of the body draws the name Hemichordata.
• Stomochord is an outgrowth from the anterior part of gut of Balanoglossus which was earlier considered as half notochord.
• Resemble the chordates in having all the three primary chordate structures, namely, gill-slits, notochord and dorsal hollow nerve cord.
• Differ from the chordates in: Lacking cephalization, metamerism, paired appendages, tail, exoskeleton, cloaca, living endoskeleton, haemoglobin and RBC.
• Containing dorsal heart
• Having open neurocoel
• Bearing numerous gonads
• Gill slits are dorsal in position instead of lateral as in chordates
• Resemble the echinoderms in nervous system, coelom and larval form. They also have common habits, ecological niche and possess remarkable power of regeneration.
• Important from evolutionary point of view as they link the chordates with non-chordates. E.g. Balanoglossus (tongue worm).

Fig. Balanoglossus : External features in dorsal view