Ordinarily speaking, the course of development is an ascending one, and the adult is more highly organised than the young; but there are cases in which there is an apparent reversal of this law, and the adult is to all appearance a degraded form when compared with the embryo. This phenomenon is known as "retrograde" or "recurrent" development; and well-marked instances are found amongst the Cirripedia and Lernaeae, both of which belong to the Crustacea.
Thus, in the Cirripedes (acorn - shells, etc.) and in the parasitic Lernaeae (fig. 3), the embryo is free-swimming and provided with organs of vision and sensation, being in many respects similar to the permanent condition of certain other Crustacea, such as the Copepods. The adult, however, in both cases, is degraded into a more or less completely sedentary animal, more or less entirely deprived of organs of sense, and leading an almost vegetative life. As a compensation, reproductive organs are developed in the adult, and it is in this respect superior to the locomotive, but sexless, larva.
Fig. 3. - A, Young of one of the parasitic Crustaceans (Achtheres) with its swimming-paddles and eye-spots, magnified; B, Deformed and swollen parasitic adult of another member of the same group (Lernoea).