TPO 43 integrated writing skill

According to the reading passage, author lists three theories to show how agnostids have lived, since, lecturer in the listening holds opposite ideas and stated other competing theories, which contradict passage's arguments.
Firstly, the essay assumes that agnostids may have been free-swimming predators that hunted smaller animals. However, the listening material claims that all of those arthropods have large eyes, but fossil remains show that agnostids had tiny eyes, relatively blind. So, they would have some other special organs.
Secondly, the writer indicates that agnostids may have dwelled on the seafloor. On the contrary, speaker points out that seafloor animals move slowly and stay in localized areas than moving to different areas. Agnostids were able to move to another areas fast, thus this characteristic is unusual for seafloor creatures.
Finally, the passage discusses that probably agnostids were parasites, because they were living on and feeding off larger organisms. In contrast, he reckons that parasites usually stay in certain limited populations, but agnostids lived in large populations, therefore, this explanation refutes the passage's argument