Delage and Herouard (Hydrozoa ) were the first to make an heroic attempt to unite the two classifications into one, to which Hickson (Hydrozoa ) has made some additions and slight modifications.
Hickson considers that the families Milleporidae and Stylasteridae should stand quite apart from one another and should not be united in one order.
Hickson, " The Medusae of Millepora," Proc. Roy.
Hickson and others, that in the bivalves Pecten and Spondylus, which also have eyes upon the mantle quite distinct from typical cephalic eyes, there is the same relationship as in Oncidiidae of the optic nerve to the retinal cells.
Hickson, "Coelenterata and Ctenophora," Cambridge Natural History, i.
(From Hickson.) a, Prae-corneal epithelium.