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THE POPULAR SCIENCE MONTHLY. 



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 lit- 

 tle barbed arrows. 



Fig. 11. Medusa of Coromorpha nutans. (Magnifled.) 



Chronologically, the Hydrm (Fig. 12) come first in the group 

 Sydroidce^ 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. 



