374 MARGARET FLOY WASHBURN 



Crustacea. Most of the work done on animals of this group 

 concerns, in one way or another, their reactions to light. Mc- 

 Ginnis (18) has observed the behavior of the fairy shrimp, 

 Branchipus serratus, tinder stimulation by light, heat, and grav- 

 ity. The animal is positively phototactic under lights from 

 twelve to two hundred and eighty candle power. After exposure 

 to darkness for a period of from twelve hours to six weeks the 

 shrimps are still positive. Light has also a kinetic effect, in- 

 creasing with its intensity; that is, it stimulates to activity. 

 When Branchipus reacts to light its ventral side is turned 

 towards the source, and its long axis is at right angles to the 

 direction of the rays. The eyes appear to be the only organs 

 for the reception of light as a stimulus. Branchipus is positive 

 to temperatures from 14 to 17 degrees C, and avoids tempera- 

 tures above and below these limits. It is positively geotropic 

 in light and negatively geotropic in darkness; light therefore 

 must have besides its directive and kinetic effects a third influ- 

 ence whereby geotropism is reversed. It is possible that dark- 

 ness rather than light may furnish the stimulus to this reversal. 



The temperature sensibility of Zoea larvae has been inves- 

 tigated by Schmid (29), who notes a distinct reaction of jump- 

 ing back given by the animals when they reach the boundary 

 between water at ordinary temperature and water at a tempera- 

 ture too high for them. 



The effect of chemicals on the reactions to light made by 

 Crustacea has been the subject of some experiments by Bohn 

 and Drzewina. Drzewina (8) finds that hydrocyanic acid is a 

 desensibilisator to light ; she notes also that copepods of the 

 plankton are much more sensitive to its influence than those 

 of the surface layers of water, where much decomposition is 

 always going on. Bohn, working with larval lobsters, finds that 

 both acids and alkalis are sensibilisators. The effect lasts longer 

 with alkalis, but is weaker. With acids, the maximum effect 

 is shown at the end of a few minutes, and the effects vanish 

 after a few hours. With alkalis, the maximum effect occurs 

 only after several hours, but the influence may persist for sev- 

 eral days. The two experimenters working in collaboration (9) 

 found that potassium cyanide reverses positive phototropism in 

 certain Crustacea and "desensibilises" to tactile stimuli more rap- 

 idly than to light. 



