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it says in my test prep book "the amphioxus and tunicates do not loose their notocord"
but I checked this in my Campbell bio text book and it says
"the adult tunicate scarcely resembles a chordate. It displays no trace of a notochord, nor is there a nerve cord or tail. Only the pharyngeal slits suggest a link to other chordates. But all four chordate trademarks are manifest in the larval form of some groups of tunicates."
so the tunicates do loose their notochord by the time they're adult. can somone please explain the discrepancy
simple: your test prep book is wrong. Now, if it had said "Amphioxus and some tunicates do not loose their notochord" then it would have been correct.
With the Cephalochordata it is easy, all species of amphioxus retain their notochord in the adult stage.
With Tunicata, the rule is like this: all of them have a notochord in their tail in the larval stage, but they loose the notochord upon metamorphosis, when they loose their whole tail. There are a few tunicates, however, that make up an exception to this rule. The Apendicularia(Oikopleura sp.) do not undergo metamorphosis, and the adult retains all the characteristics of the larva, including the notochord. But it is simply an exception, not the general rule.
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter
But it might be a very important exception indeed.
It is of course vain and superfluous to add anything to the message of an all-powerful moderator , especially when the answer of the moderator is absolutely right (which he is), but I will - respectfully - give it a try anyway.
In textbooks, including very recent ones, Cephalochordates (amphioxus) are considered as the closest group from Vertebrates, (the fact is that indeed an amphioxus does look a lot more like a sort of fish than an adult tunicate does), the Tunicates being considered themselves as the closest from the [Vertebrates + Cephalochordates] group.
However, recent molecular data (gene sequencing) casts doubt on this interpretation. If these data are confirmed, Tunicates, not Cephalochordates, would be the closest group ("sister group") from Vertebrates. The Cephalochordates would therefore be "only" the sister group to the group [Vertebrates + Tunicates].
Why bothering with that detail ? Because it is relevant to your question: it is suspected that Vertebrates would have arisen more specifically from the Apendicularia because, as MrMystery pointed it out, they keep their chord even as adults. The scenario is then that some tiny group of Apendicularia-like creatures evolved into a large and (to this day) successful group: the Vertebrates.
France (another small and unimportant country like Romania )
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