I42 THE HISTORY OF CREATION, 
of three different classes, namely, the Hood-jellies (Hydro- 
medusz), the Comb-jellies (Ctenophora), and the Corals 
(Coralla). The hypothetical, extinct Archydra must be 
looked upon as the common primary form of the whole 
group; it has left two near relations in the still living 
fresh-water polyps (Hydra and Cordylophora). The 
Archydra was very closely related to the simplest forms 
of Spongiz (Archispongia and Olynthus), and probably 
differed from them only by possessing nettle organs, and 
by the absence of cutaneous pores. Out of the Archydra 
there first developed the different Hydroid polyps, some 
of which became the primary forms of Corals, others the 
primary forms of Hydromeduse. The Ctenophora de- 
veloped later out of a branch of the latter. 
The Sea-nettles differ from the Spongiz (with which 
they agree in the characteristic formation of the system of 
the alimentary canal) principally by the constant posses- 
sion of nettle organs. These are small bladders filled with 
poison, large numbers—generally millions—of which are 
dispersed over the skin of the sea nettles, and which burst 
and empty their contents when touched. Small animals 
are killed by this; in larger animals this nettle poison 
causes a slight inflammation of the skin, just as does the 
poison of our common nettles. Any one who has often 
bathed in the sea, will probably have at times come in con- 
tact with large Hood-jellies (Jelly-fish), and become ac- 
quainted with the unpleasant burning feeling which their 
nettle organs can produce. The poison in the splendid blue 
Jelly-fish, Physalia, or Portuguese Man-of-war, acts so 
powerfully that it may lead to the death of a human being. 
The class of Corals (Coralla) lives exclusively in the sea, 
