Final answer:
The whip-like motion in nematodes is due to their longitudinal muscles contracting against the pseudocoelom, coordinated by their nervous system, resulting in side-to-side undulations.
Step-by-step explanation:
The whip-like motion in nematodes is primarily due to the unique structure of their nervous and muscular systems. Nematodes have four longitudinal nerve cords running along the length of their body, with a better developed ventral nerve cord. The nerve cords allow for the coordination of muscle contractions. Also critical to their movement is their muscular layer, which consists solely of longitudinal muscles underneath the epidermis. This means nematodes can only produce side-to-side undulations, resulting in the characteristic whip-like motion as they move.
Nematodes lack circular muscles, which in other organisms would permit other types of movement. The longitudinal muscles contract against the nematode's high-pressure fluid-filled pseudocoelom, which functions as a hydrostatic skeleton. The controlled contraction of these muscles on alternating sides of the body produces a thrashing or whipping motion that propels the nematode forward or helps it to navigate through its environment.