1. In Leptosynapta inhœrens autotomy occurs regularly when unfavorable conditions of environment arise. Experiments with Thyone briareus did not show more than 35 per cent. of cases of autotomy, even under the most favorable circumstances for its occurrence. 2. The mechanism involved in autotomy is quite different in the two genera. Thyone breaks the longitudinal muscles and throws out the visceral organs at the anterior end. Leptosynapta constricts off pieces at the posterior end of the body. 3. Portions of the body which have been separated from the anterior region, which contains the nerve ring, do not show autotomy. 4. Strong stimuli which bring about a very active or a strongly contracted condition do not always induce autotomy.