[Ultrastructure of optic centers of the crayfish Artemia salina].

In electron microscopic investigations of the eyestalk of A. salina we were not able to find any elements of the most important neuroendocrine system X-organ--sinus gland which is typical of higher Crustaceans. The data obtained indicate that this system appears only in higher Crustaceans as the result of development of their central nervous system.