ROESEL VON ROSENHOF 301 



A bowl covered with duckweed was brought to him, 

 and on searching the duckweed he found abundance 

 of the long-desired Hydra. He was now able to study 

 the structure and activities of the polyp, and to dis- 

 criminate the various species. These investigations 

 gave him full employment during months of seclusion. 



Trembley had seen the ovary of Hydra and noted the 

 discharge of the eggs, but had not clearly traced the 

 development of the polyp to the egg. Koesel took 

 much trouble to complete the life-history, but without 

 altogether succeeding. 



The floating duckweed also yielded a freshwater polyp 

 of a very different kind, which resembled Trembley's 

 "polype a panache" and Baker's " bellflower-animal- 

 cule,"^ but differed in various details, and especially 

 in the mode of branching. The figures of the three 

 naturalists now make it clear that Trembley and Baker 

 had before them the polyzoan called Lophopus ; Eoesel's 

 polyzoan was a Plumatella. The small rounded particles 

 about as big as pins' heads, which developed plumes, 

 and which Roesel calls " der kleinere Federbusch-polyp," 

 are now recognised as the young of the polyzoan 

 Cristatella.^ 



We cannot fail to admire the enthusiasm and industry 

 of Roesel, his delight in the study of living things, and 

 his skill as a draughtsman. But we must not omit to 

 mention that his judgment is often unsound, and that he is 

 too fond of putting forth what can only be called guesses. 

 He was, like nearly every naturalist of the eighteenth 

 century, an untrained amateur, and as he had engaged 

 to supply monthly descriptions of unfamiliar animals, it 

 is not surprising that he should now and then publish 



» Employment for the Microscope (1763), pi. XII, figi. 16-22. 

 2 Allman, Freshwater Polyzoa, p. 77. 



