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Hemichordata and Chordata

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Hemichordata and Chordata - Lesson Summary

Earlier, this phylum was considered a sub-phylum under chordates. However, due to certain features, taxonomists have classified this phylum as non-chordates.

Hemichordates are small, worm-like marine animals. They are bilaterally symmetrical, triploblastic, coelomate animals with an organ-system level of organization.

Their cylindrical body is divided into proboscis, collar and trunk. Hemichordates respire through gills and have an open circulatory system. The proboscis gland functions as an excretory organ. Hemichordates are dioecious and fertilisation is external with indirect development. Balanoglossus and Saccoglossus are examples of this phylum.
 
Phylum Chordata:

Chordates are animals with a notochord, a dorsal, hollow nerve cord, paired pharyngeal gill slits and a post-anal tail.

They are bilaterally symmetrical, triploblastic, coelomate animals with an organ-system level of organisation and a closed circulatory system.

Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata and Vertebrata.

Subphyla Urochordata and Cephalochordata are also referred to as protochordates. These animals are found only in marine habitat. Urochordates have a notochord only in the larval tail. While cephalochordates have a persistent notochord, which extends from head to tail region.

Ascidia and Doliolum are examples of urochordates, while Branchiostoma is an example of cephalochordates.

Members of subphylum Vertebrata possess a notochord during the embryonic period, which is later replaced by a cartilaginous or a bony vertebral column.

Vertebrates have a two, three or four-chambered heart for circulation of blood, kidneys for excretion and osmoregulation and paired appendages which could be either fins or limbs. 

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