Fig. 1. Experimental arrangement for recording digging behaviour (A) and the method used to measure segmental deflection (B). (A) L-shaped acrylic walls were placed in an aquarium to induce digging behaviour. Solid gelatine served as a substratum. The sea water was aerated and kept at 14°C. (B) Points a and b indicate the anterior and posterior margins of the head, respectively. Similarly, points c and d indicate the posterior margins of the second and third thoracic segments, respectively. For the second thoracic segment, lines ab and bc formed the angles of extension and flexion. For a more detailed explanation, see the text.