The Hydrozoa are thus shown to belong to the group of Coelenterata Cnidaria, and it remains to consider more fully their distinctive features, and in particular those which mark them off from the other main division of the Cnidaria, the Anthozoa, comprising the corals and sea-anemones.
2, "Coelenterata," by Carl Chun, by permission of C. F.
This is a process of gastrulation by invagination which is found in all animals above the Coelenterata, but which is very rare in the Cnidaria, and is known only in the Scyphomedusae amongst the Hydrozoa.
GENERAL WORKS ON HYDROZOA.-C. Chun, "Coelenterata (Hohlthiere)," Bronn's Klassen and Ordnungen des Thier-Reichs ii.
Hickson, "Coelenterata and Ctenophora," Cambridge Natural History, i.