首页>英语角 > 听力 > 美音 > 课 程 > 科学 > 博士>What Is Phylum Chordata?
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Hi, I'm Holly Deambrosi for About.com. What do fish, amphibians, reptiles, birds and mammals have in common? They all belong in the phylum chordata, members of which are also known as chordates.All chordates are characterized by the presence of a notochord, or internal, supporting skeletal rod, in some stage of their development. In some cases the notochord is present only in the embryonic stage. Most chordates are vertebrates, but there are also two categories of invertebrate chordates.Some of these features include the presence of bilateral symmetry, an endoskeleton, body segmentation, a complete digestive system and a tail. All chordates have bilateral symmetry. For example, if a line is drawn directly down the middle of this frog, you will notice that it is essentially divided in half with each side being symmetrical, or a mirror image of the other.This internal, supporting framework may be bony or cartilaginous, such as in sharks or young vertebrates. Body segmentation is simply the division of the body into regions, such as the head, trunk, legs and feet. The presence of a complete digestive system in chordates indicates two openings for the digestive tube so that food and waste do not mix. The tail may be present only in embryonic form, as in tadpoles, otherwise this feature is present through the adult stage.So now you know some of the basics about chordates, can you think of more examples of phylum chordata? Which of these examples is a chordate? (sea turtle, sea star, jellyfish) This one should be pretty easy. Look at the sea star and the jellyfish. Both of these animals have radial, not bilateral, symmetry. Neither has a tail or formal body segmentation. So we know right away that neither of these is a chordate. The sea turtle is a vertebrate, has a tail, a complete digestive system, body segmentation, is bilaterally symmetrical, and is classed as a reptile. The sea turtle is a chordate.Thanks for watching! To dive deeper into the fascinating study of phylum chordata, visit About.com.Special thanks to www.minimiraclesfarmsc.com
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