190 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



float it out in water under a dissecting microscope. The buoyant 

 power of the water will lift the larva or adult into view above the 

 slime and teeth, particularly if the dish be agitated a little. 



The living material upon which the following observations were 

 made was obtained at Lake Maxinkuckee, Indiana, during the sum- 

 mers of 1906, 1908, and 1909, while the author was in the employ 

 of the U. S. Bureau of Fisheries. For this valuable opportunity 

 acknowledgment is gratefully made to the Hon. George M. Bowers, 

 U. S. Commissioner of Fish and Fisheries. The serial sections and 

 the study upon them were made in the biological laboratory of Johns 

 Hopkins University, and the sincere thanks of the author are due to 

 Dr. E. A. Andrews for many valuable suggestions and corrections. 



METHODS. 



For external study, including the mouth-parts and other append- 

 ages, a mixture of 95 per cent alcohol and 5 per cent formalin in equal 

 parts was found to be the most satisfactory preservative. Speci- 

 mens kept in this mixture for three years have retained their ana- 

 tomical form and structure perfectly, and have become neither unduly 

 hard nor brittle. 



For the histological work the material was preserved in alcoholic 

 corrosive-acetic, the corrosive being removed immediately after fix- 

 ation with iodine. 



The specimens to be sectioned were first stained in bulk with Del- 

 afield's hsematoxylin, and after clearing were counterstained with 

 eosin in 95 per cent alcohol. 



These methods were found to give excellent results both in fixa- 

 tion of the tissues and in differential staining. 



HISTORICAL. 



The genus Achtheres was established by Nordmann in 1832, with 

 the type species Achtheres fercarum, found in great abundance on 

 the gill arches of the common European perch, Perca jiuviatilis. 

 Nordmann not only described the adults of both sexes minutely, but 

 he also gave a good account of the breeding habits and the develop- 

 ment up to the first copepodid stage. In the same paper he estab- 

 lished another genus, Tracheliastes, closely related to Achtheres. He 

 did not obtain the male of his type-species, Tracheliastes polycolpus, 

 but did secure some newly hatched larvse, whose development he 

 also followed up to the first copepodid stage. In a third new genus, 

 Basanistes, with the type-species huchonis described at the same time, 

 he was not so fortunate, and no developmental stages are mentioned. 



Three years later (1835), however, Kollar reinvestigated this third 

 species and included with the adults a good description and figures 

 of the first copepodid larva. 



