Hadley, Behavior of the American Lobster. 277 



4. Rotations in the radii of a circle — In these the longitudinal 

 axis of the larva formed a radius, and with either the head or the 

 tail at the center the animal rotated about a fixed point. These 

 reactions were uncommon and, as yet, unexplained. 



These four types of movement seldom occurred separately, 

 except under especially devised experimental conditions. Under 

 natural conditions, they were usually combined to form a compos- 

 ite action. To the previously mentioned simple components, how- 

 ever, all the more complex movements of the larval lobsters could 

 be reduced. 



B. The Effect of Screens and Backgrounds — It is prob- 

 able that the reactions which are brought about through the use 

 of backgrounds, are, generally speaking, dependent upon the same 

 factors and conditions of illumination which are effective when 

 light-absorbing or light-scattering screens are used. The term 

 *' screening" has been employed by Bohn (1905) to designate his 

 method of submitting organisms to the influence of surfaces of 

 light and shade. This investigator made use of screens of black 

 and white of such size that he could readily bring them close to the 

 sides of the glass containers in which the organisms under observa- 

 tion were placed. He has made a special study of the reactions of 

 Crustacea to the influence of such screens, and in several instances 

 the observations of the writer upon the larvae of Homarus ameri- 

 canus merely confirm certain points in Bohn's earlier work. In 

 many instances, however, new facts have been added. 



The influence of white screens — The lobster larvae were confined 

 in a cylindrical jar, crystallization dishes, or in a rectangular glass 

 container. The latter was used most frequently. The larvae 

 were then placed in the dark box and this was illuminated in such 

 a manner that a general twilight was produced and the directive 

 influence of light was at a minimum. While making observations 

 it was even found necessary that the writer should wear a black 

 mask over his face and collar, and, often, darken his hands in 

 order not to modify the uniform light. For white screens pieces 

 of white cardboard were employed, and brought over, under, or 

 beside the receptacle containing the larvae, as the case might re- 

 quire. Sometimes the screen was brought gradually toward the 

 container, sometimes abruptly; but in all cases the results were 

 definite and agreed with great uniformity. In order to secure the 

 best results with the white screen, it was found best to reduce the 



