basal disk

I have only seen one instance of what appeared to be unequal vertical fission—that of a polyp consisting of two individuals still joined together by the basal disk, but one about half the size of the other.

In doing so it bends its column down to the object along which it is about to move in such a way that it lies almost parallel to the surface, the basal disk, however, being still attached.

The basal disk is liberated and the tentacles contract, dragging the column, which still lies prone, along as they do so.

The basal disk is then turned round and fixed to the new support.

The column is dragged forward, still lying almost prone, the basal disk is bent downwards and again attached, and the whole movement is repeated.