Chapter 11 HEMICHORDATA & CHORDATES

HEMICHORDATA & CHORDATES

EVOLUTIONARY PERSPECTIVE

Hemichordata have some most familiar animals. We are familiar with them because we are members of phylum Chordata. Other members of these phyla are much less familiar. Some hemichordates form coiled castings (sand, mud, and excrement) at the opening of  U-shaped burrows. A wormlike animal comes out of burrow. This wormlike animal is a member of a small phylum hemichordata. Other members or this phylum are unfamiliar filter feeders, they are called pterobranchs.

Sonic hemichordates cling to rocks exposed by low tide. They look like jellylike masses. They have two openings at their unattached end. Some live as solitary individuals. Others live in colonies. If these animals are touched, a stream of water comes out from their openings. Normally it is difficult to recognize them as small filter feeders. They are sea squirts or tunicates. Tunicates and cephalochordates lack a vertebral column. Therefore, they are called the invertebrate chordates.

PHYLOGENETIC RELATIONSHIP

Hemochordata, Chordata and Echinodermata are deutrostomes. Therefore,  zoologists believe that they have some common triploblastic ancestor. But this fossil could not be discovered. The chordates have following characteristics:

1. They have dorsal tubular nerve cord.

2. Notochord is present in them.

3. They have pharyngeal gill slits or gill punches.

4. They have post anal tail.


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Conclusion: Gill slits are the only character which is present in Hemichordates. Dorsal nerve chord is also present in some species. Therefore, most zoologists believe that there are closer evolutionary ties between chordates and Hemichordates than the Echinoderms. But Chordates and Hemichordates have arised from different points in Deutrostome linage.

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