Final answer:
The obvious characteristic of body form that can be used to 'slot' the six different animals into 4 groups is symmetry.
Step-by-step explanation:
The obvious characteristic of body form that can be used to "slot" the six different animals into 4 groups is symmetry. Symmetry refers to how the body parts are distributed along the major body axis. There are three types of symmetry: asymmetry (lack of symmetry), radial symmetry (body parts arranged around a central point), and bilateral symmetry (body can be divided into roughly equivalent halves along one axis).