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.
` tion of the coelenteric 1 V cavity; in the second stage the tentacles Much modified from C. Chun, " Coelenterata," in grow out as secondary Bronn's Tierreich.
Milne-Edwards removed the Polyzoa; the group was soon further thinned by the exclusion of the Protozoa on the one hand and the Entozoa on the other; while in 1848 Leuckart and Frey clearly distinguished the Coelenterata from the Echinodermata as a separate sub-kingdom, thus condemning the usage by which the term still continued to be applied to these two groups at least.
In 18J5, however, Owen included under Lamarck's term Radiaria the Echinodermata, Anthozoa, Acalepha and Hydrozoa, while Agassiz also clung to the term Radiata as including Echinodermata, Acalepha and Polypi, regarding their separation into Coelenterata and Echinodermata as "an exaggeration of their anatomical differences" (Essay on Classification, London, 1859).
These attempts, however, to perpetuate the usage were finally discredited by Huxley's important Lectures on Comparative Anatomy (1864), in which the term was finally abolished, and the "radiate mob" finally distributed among the Echinodermata, Polyzoa, Vermes (Platyhelminthes), Coelenterata and Protozoa.
To these three characters the Hydrozoa add a fourth which is distinctive of the subdivision of the Coelenterata termed the Cnidaria; that is to say, they always possess peculiar stinging organs known as nettle-cells, or nematocysts (Cnidae), each produced in a cell forming an integral part of the animal's tissues.
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.
2, "Coelenterata," by Carl Chun, by permission of C. F.
Hickson, "Coelenterata and Ctenophora," Cambridge Natural History, i.