A medusa with a remarkable habit of life is Mnestra parasites, which is parasitic on the pelagic mollusc Phyllirrhoe, attaching itself to the host by its subumbral surface; its tentacles, no longer required for obtaining food, have become rudimentary.
Thus in the primitive mollusc the mantle-cavity contains a symmetrical group of structures at the posterior end of the body, and this group of structures is called the pallial complex.
It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod.
He regarded it as a mollusc, being no doubt deceived by the slug-like appearance given by the antennae.
So far the young mollusc has not reached a definitely molluscan stage of Xviji.