Sub-Phylum : Hemichordata
Classification of Animal Chordates of Class 11
- 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