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GENERAL CHARACTERS AND CLASSIFICATION OF HEMICHORDATES

Published in Zoology
Monday, 05 June 2017 22:26
Balfour classified urochordata, cephalochordata and vertebrata as phy­lum chordata. But Bateson included Hemichordata in this phylum chordata. These members are considered as primitive chordata.
 
  1. The body of these animals is long and worm like.
  2. These are marine animals. They make burrows in the coastal sandy shores.
  3. Their body is divisible into proboscis, collar and trunk.
  4. They emit strong idoform smell.
  5. Their body is covered by ciliated epidermis.
  6. Their body wall is muscular.
  7. They show enterocoeliccofelome. This coelome is divided into 5 cavities, one in the proboscis, two in the collar and two in the trunk region.
  8. In the proboscis buccal diverticulum is present. It is called stctnochord. It is comparable to notochord.
  9. In these animals excretory organs are absent. In the proboscis glomerulus is present. It will help in excretion.
  10. Digestive system is well developed. The alimentary canal is long. It starts with mouth and ends with anus. Pharynx is useful for food collection and respiration. These are filter feeders.
  11. On the lateral sides of Pharynx gill slits are present.
  12. Blood vascular system resembles annelids, blood vascular system more.
  13. Heart and Pericardium are present in the proboscis.
  14. In the collar lateral blood vessels are present.
  15. Nervous system includes a dorsal tubular nerve cord and solid ventral nerve cord.
  16. These animals will not show brain and cranium. Hence this is grouped aeraniata.
  17. These are unisexual animals gonads will give gonoducts.
  18. Fertilization is external.
  19. Holoblastic clevage is seen.
  20. In the life history a larval form is seen. Tornaria larva is usually seen.
 
CLASSIFICATION:
 
Hemichordata subphyllum is classified into two classes.
 
  1. Enteropneusta
  2. Pterobranchia
 
Class 1: Enteropneusta:
 
  1. These animals are worm like and cylindrical.
  2. They are burrowing animals.
  3. They show many pairs of gills.
  4. Alimentary canal is long and straight. Ex: Balanoglossus, Ptychodera.
 
Class 2: Pterobranchia:
 
  1. They lead sedentary life.
  2. They are tubicolous.
  3. They are colonial organisms.
  4. They show less number of gills.
  5. Alimentary canal is 'U' shaped. Ex: 1. Cephalodiscus 2. Rhabdopleura.

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