Step 1: What a larval form tells us.
Many animals pass through a special young swimming stage called a larva before becoming an adult. These larvae are often unique to a particular group, so they act like clues to identify the group.
Step 2: Recall the Tornaria larva.
Tornaria is a ciliated, free-swimming larva. It is famous because it looks very similar to the bipinnaria larva of echinoderms, which hints at a close link between the two groups.
Step 3: Which group owns Tornaria.
The Tornaria larva belongs to Hemichordata, the group that includes Balanoglossus, the acorn worm.
Step 4: Why the other options are wrong.
Echinodermata have larvae such as bipinnaria, not Tornaria. Cephalochordates like Amphioxus and urochordates like Herdmania have their own tadpole-like larvae, not Tornaria.
Step 5: Conclusion.
Therefore the correct answer is "Hemichordata". \[ \boxed{\text{Hemichordata}} \]