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(1) The asymmetrical development in Hydroides opercula is due to the growth difference between two sides in the same kind of organs. The presence of a functional organ holds its mate in check. Therefore, taking off the functional operculum results in the development of the rudimentary side. (2) This phenomenon of compensatory regulation cannot be considered to be brought about by a nervous reflex set in motion by the stimulation of damaged nerves in the operated operculum, since, for example, the inhibitory influence to a certain extent depends on the degrees of injury. (3) It is also not proved to be due to the production of a certain chemical substance which stimulates the rudimentary organ to grow, while the wound itself affects directly the inhibitory action of the functional operculum. (4) The inhibitory influence is only localized in the operculum and not outside of it. Therefore, cutting off the gill filaments or other parts of the body does not bring a reversal of asymmetry. (5) Within the limit of the operculum there is no special centre of the inhibitory influence, which depends on the intactness of the internal constitution of the organ. Those peripheral parts outside the limit of direct blood supply can be cut off, without the resulting reversal of asymmetry. A simple puncturing with a fine needle also has no effect. But greater injury accompanied by an outflow of blood always starts the rudimentary organ developing. (6) The functional operculurn has no regenerative capacity, while the rudimentary one still has. The injured operculum is thrown off sooner or later from the body, and a new start repeats from the beginning the original development. By accelerating this regenerative development of the old functional organ, or properly controlling the growth of the rudimentary organ on the opposite side, a symmetrical animal with two large opercula can easily be derived. On the other hand, if both rudiments antagonistically check each other's development, a symmetrical animal with two small rudimentary opercula is produced. (7) Since, however, either the right or the left or even both rudiments can be made to develop into a functional organ, the inhibitory influence must act at first on both alike, unless a threshold value is assumed. In the present species of Polychsete it is as a rule the right-handed one that grows more quickly than the other. (8) With regard to the nature of the asymmetry in Hydroides it seems to be quite different from that in the male Gelasimus in which the asymmetry of the chelas is said to be derived from the symmetrical condition relatively late in development, and the relation, once attained, becomes fixed. (9) In Hydroides norvegica at least, the opercula show a tendency to right-handed growth from the beginning of their development, but this condition of asymmetry is very unstable and is easily reversible by an external influence even in the adult state, i.e. there is no critical stage at which it is irreversibly determined.