The articular capsule or ligament, of the primary spines of the sea-urchin Eucidaris tribuloides behaves as a classical excitable tissue, responding with a shortening or contracture to a variety of stimuli including the cholinergic agonists acetylcholine (ACh), methacholine (MeACh), carbamylcholine (CCh) and nicotine. 2. d-Tubocurarine failed to decrease the amplitude of the acetylcholine-induced contractures, while the contractures elicited by methacholine were blocked by atropine. Neostigmine, but not eserine, increased the amplitude of ACh-induced contractures, suggesting the presence of AChE in the preparation. 3. The cholinergic agonists induced contracture of the ligament, but had quite different kinetics, the rate of rise of the contracture being fastest for ACh and decreasing in the following order: CCh, MeACh and nicotine. 4. Tyramine and octopamine exert an inhibitory action on "catch" and a relaxing effect on the contracting ligament. 5. The time courses of the contractures elicited in the same preparation were virtually identical to those of "catch". Therefore, we propose that "catch" and contracture are only two different aspects of the same phenomenon; namely the contracture of the muscle fibers present in the ligament.
