The unmovable mouth
Although they are extinct, we do kmow about the soft parrs of the ortracoderms brain, heart, gills, intestine, sence organs because they left impraints on the inner surface of their armor-plating, and this fossilized perfectly.Study of these soft parts has revealed that these creatures of almost haft a billion years ago already had the typical anatomicial organization of the vertebrates, with rhe same apparatus and sence organs. But their mouth was very primitive, unmovanle, circular usable only for sucking in seawater and filtering the organic substance in it.
This poor form of nourishment placed very severe restrictions on the development placed very severe restrictions on the development if their body. Generally speaking, the ostracoderms never grew much longer than a few inches. But their anatomical organization offset that.