A group that is often considered among the vertebrates but is thought by some scientists to be a sister group to the vertebrates includes the _______, which produces large amounts of slime as a defense mechanism, has three small accessory hearts, and does not possess stomach or jaws.

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The characteristics listed, such as large amounts of slime as a defense mechanism, three small accessory hearts, and the absence of stomach or jaws, all point towards the hagfish as the group being described. Lampreys are also considered a sister group to vertebrates, but they have a different set of characteristics such as a sucking mouth and the ability to parasitize other fish. The other choices - eel, slug, and salp - are not related to this group in any way.

A Group That Is Often Considered Among the Vertebrates but Is