The morphology of the group thus falls naturally into four sections - (I) the hydropolyp, (2) the polyp-colony, (3) the hydromedusa, (4) the medusa-colonies.
Since, however, medusa-colonies occur only in one group, the Siphonophora, and divergent views are held with regard to the morphological interpretation of the members of a siphonophore, only the first three of the above sub-divisions of hydromedusa morphology will be dealt with here in a general way, and the morphology of the Siphonophora will be considered under the heading of the group itself.
W, The parasitic hydromedusa Mnestra, usually found attached in this position by the aboral pole of its umbrella.
4501, serves to show that Trichoplax is the planulalarva of a Hydromedusa.) A, a small specimen drawn from life (X 20).
A scyphomedusa is distinguished from a hydromedusa chiefly by the following points.