HYUROTROPISMS OF FRESH-WATER INVERTEBRATES. 37 



has -upplied only thi- suggests that very little attention has been 

 paid to the topic. This may be because no simple means of 

 iM\e-tiu,itin^ it has been published. The purpose of this papi-r 

 o supply that lack and to apply the method to the study of a 

 few forms selected from different strata of the water. As types 

 .I" form- tint creep along the bottom of the water or along vege- 

 tation but <!<> not leave the water, I selected a snail [Planorbis 

 < I/i-lisoma') antrosnm Con.] and a dragon-fly nymph. As ex- 

 amples of form- that spend most of their time on the bottom, but 

 which, at times, leave the water, I have sleeted the crayfish 

 [Cambarns (Faxon i us) propinquns Girard] and a scaver 

 beetle (Tr<>/>i Remits sp. ?). As types of forms that live on the 

 surface of the water, I have selected the giant water st rider 

 (Gerris remigis Say), which, except during the hibernating season, 

 is practically a permanent inhabitant of the film, and the whirl; 

 beetle (Gyrinns sp.?) which occasionally dives and which mi- 

 cs from pond to pond. 



TECHNIQUE. 



To be able to study the movements of animals with a cer- 

 tainty that the movements were not influenced by either gravi- 

 tation, the sun's rays, chemical stimuli or contact stimuli, I 

 devised what may be appropriately called a checker-board 

 plate. This consists of a board twenty inches wide and twenty- 

 four inches long which is covered with white oilcloth that has 

 been subdivided, by printed lines, into one inch squares (Fig. l). 

 The four edges of the board were bisected by straight lines which 

 crossed, at right-angles at the center of the board. Diagonal 

 lines, bisecting these central right angles extended outward to 

 the edges of the board. The tips of the four lines mentioned 

 were named; o degrees, 45 degrees, 90 degrees, i; v s decrees, 180 

 degrees, 225 degrees, 270 degrees, 315 degrees. By means of a 

 level and leveling devices this board wa- made perfectly hori/on- 

 tal. Being horizontal, gravitation could not influence the move- 

 ments. Being an absolutely smooth surface, the movements 

 could not be influenced by contact -timuli. I'.ein- c .\en-d with 

 oil-cloth, it was possible, by \va-hin-, to keep ii free from chemical 

 -tiinnli. It was protected from the sun by means of a screen of 

 -ort; this made it impossible for the -un'- ray- to influence 



