The iris is in most young birds at first brown or dull-coloured, but with maturity attains often very bright tints which add considerably to the charm of the bird; sexual dimorphism is in this respect of common occurrence.
Molluscs are usually of separate sexes, but sexual dimorphism is seldom highly developed.
In some species of Dinophilus there is pronounced sexual dimorphism (the male being small and without gut) as in the Rotifera.
Dimorphognathus, from West Africa, is the unique example of a sexual dimorphism in the dentition, the males being provided with a series of large sharp teeth in the lower jaw, which in the female, as in most other members of the family, is edentulous.
Moreover, this is nullified by extreme sexual dimorphism.
