nonsegmented

Nonsegmented is an adjective used in biology to describe organisms or anatomical structures that do not exhibit a regular repetition of body units along their length. Segmentation refers to a serial repetition of modules or segments, so nonsegmented organisms lack this repeated unit structure in their external body plan.

In the animal kingdom, many phyla include nonsegmented forms. Examples commonly cited are cnidarians (such as jellyfish and corals) and flatworms (Platyhelminthes), as well as many nematodes. By contrast, segmented animals include groups such as annelids (earthworms and their relatives) and most arthropods, where the body is composed of distinct, repeated segments. In higher vertebrates there is internal segmentation in the form of somites during embryonic development and in the vertebral column, even though the external body may not display obvious, repeated external segments.

The presence or absence of segmentation has functional and evolutionary implications. Segmentation can facilitate regional specialization, modular growth, and varied locomotor strategies. Nonsegmented body plans may be advantageous for certain life histories, defensive strategies, or developmental constraints where a uniform body plan is favored. Some taxa exhibit partial or internal segmentation without corresponding external segmentation, sometimes described as pseudosegmentation or tagmosis.

In broader usage, nonsegmented can apply to structures or sequences that lack division into segments, though in scientific contexts it is most often used to contrast with clearly segmented body plans in anatomy and developmental biology.