Laterigrade

Laterigrade is an anatomical term used in zoology to describe limbs oriented to the side of the body rather than along the forward axis. Laterigrade appendages typically project laterally and bend toward the side, enabling locomotion that is more sideways than forward and often facilitating gripping or clinging on irregular substrates.

The term is most commonly applied to the limbs of crustaceans, particularly decapods such as crabs and some lobsters, whose walking legs (pereiopods) extend from the sides of the thorax. In these animals the pereiopods are arranged in a lateral plane and move predominantly sideways, which supports rapid sidestepping and stable stance on uneven surfaces. In contrast, orthograde limbs are oriented so the joints and segments move primarily forward.

Beyond crustaceans, laterigrade limb arrangements have been observed in a variety of arthropods in the fossil record and in some extant groups, and can have functional implications for defense, maneuverability, and habitat use. The term is a descriptive one rather than a taxonomic group, and its presence is primarily noted as a morphological feature rather than a diagnostic character for a single lineage.

Etymology: from Latin latus “side” and gradus “step” (with the -grade suffix indicating a direction of movement). The term is used in comparative anatomy to contrast limb orientation and locomotor strategy across species.