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 animals described are Protochordates, a group comprising subphyla like Cephalochordata and Urochordata. These organisms are exclusively marine, exhibit bilateral symmetry, and possess a true coelom. A defining characteristic of Protochordates is the presence of a notochord at some stage of their life cycle, which provides structural support. However, unlike members of the subphylum Vertebrata, Protochordates never develop a bony or cartilaginous vertebral column; their notochord is not replaced by a backbone [1]. For instance, in lancelets (Cephalochordata), the notochord persists throughout life from head to tail, while in tunicates (Urochordata), it is typically present only in the larval tail. While they belong to the phylum Chordata, the specific group that lacks a vertebral column entirely is referred to as Protochordata [1].

Sources

1. [1] https://www.elearning.raghunathpurcollege.ac.in/files/05DE960B16324076580.pdf