Abstract
Transmission electron microscopy revealed varying morphology of the dorsal part of the duct indicating the occurrence of different stages of activity. In some animals the tip cavity contains an amorphous substance which, at another phase of activity, is released into the surrounding hemocel through interstices in the walls of the tip. A transitional zone separates the dorsal tip from the large ventral part of the duct, where the wall, near the transitional zone, consists of a single layer of thin cells joined by tight junctions. Ventrally, the large ciliated cells lie in two levels near the opening into the buccal cavity. They bear numerous, tightly packed cilia which beat vigorously in living animals. The organization of this part resembles that of protonephridia. An excretory function is suggested for this part of the duct.
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