Animals which are marine, bilaterally symmetrical, have a coelom and a notochord, but never form a vertebral column are placed under which one of the following groups?
The description fits the characteristics of Protochordata, which include Urochordata (Tunicates) and Cephalochordata (Lancelets). These are marine, bilaterally symmetrical coelomates that possess a notochord at some stage of their life cycle, but they do not develop a vertebral column (backbone), which is characteristic of Vertebrata.
Protochordata is a group within the phylum Chordata characterized by having a notochord but lacking a vertebral column.
The phylum Chordata is broadly divided into Protochordata (also treated as subphyla Urochordata and Cephalochordata) and Vertebrata. Vertebrata are chordates where the notochord is replaced by a bony or cartilaginous vertebral column in the adult stage. Mammalia is a class within Vertebrata.