What term describes a body plan that lacks a definite shape?

The term that describes a body plan that lacks a definite shape is asymmetrical. Asymmetry refers to organisms whose body structure is not uniform or is unequally divided, meaning they do not possess a mirror-image symmetry that many other forms of life exhibit. This term is often applied to certain animals, like sponges and some forms of jellyfish, which do not have a consistent shape and can appear irregular or varied in form.

In contrast, symmetrical body plans are characterized by regular arrangements that result in mirror-image halves, such as bilateral symmetry, where the left and right sides are mirror images of each other, or radial symmetry, where body parts are arranged around a central axis. Linear body plans describe organisms that appear elongated or straight, typically seen in certain types of organisms but do not relate to the lack of shape.