Final answer:
Salps in the class Thalacea are filter feeders, and Pikaia gracilens, the earliest known chordate, closely resembles a lancelet.
Step-by-step explanation:
Salps, belonging to the class Thalacea, are filter feeders that feed on phytoplankton. They do not have a hard exoskeleton, nor do they exhibit radial symmetry, and they are not members of the phylum Cnidaria; instead, they are classified under the phylum Chordata. Pikaia gracilens, collected from the Burgess Shale of British Columbia and recognized as the earliest known chordate, most closely resembles a lancelet. Lancelets are simple chordates that share significant characteristics with the early chordates and possess the basic features of this phylum, which are not found in the other options such as hagfish, lamprey, or tunicate.