seemed an invisible sheet of living fire. A tentacle of the hydroid, when carefully pressed between two glass slides, or in a compressorium, may be seen, under the microscope, to dart out thousands of these little barbed arrows.
Chronologically, the Hydrœ (Fig. 12) come first in the group Hydroidœ for they were first carefully studied and truly classified by Trembley. His observations, though made in the earlier half of the eighteenth century, were so accurate, and his delineations so correct.