The phenomenon of seasonal dimorphism is of especial moment for the plankton dwellers.
In some species of Dinophilus there is pronounced sexual dimorphism (the male being small and without gut) as in the Rotifera.
After this a marked sexual dimorphism sets in.
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.