RHEOTAXIS IN ISOPODA 285 



was molted piecemeal and was not entirely finished until nine 

 days later. In another instance the antennae and head shed 

 their covering although the general molt did not occur until 

 seven days later. 



The general effect of the molting period upon rheotaxis is 

 shown in table 9; it is best illustrated by the following 

 account taken from notes made at the time of the first observed 

 molting of isopod no. XI (see also charts 5 and 5 a). This 

 isopod was a male, 12 mm. long, which averaged normally 86 

 per cent positive reactions. The molt began just after the first 

 test of eleven individual trials which resulted in a response of 

 35 per cent positive, 35 per cent negative and 30 per cent indefi- 

 nite, with an efficiency of 2.5. 



The covering broke between the fourth and fifth thoracic seg- 

 ments and the forward part was worked off over the head by 

 a series of undulating motions of the body and by movements 

 of the legs, both the posterior ones and those immediately con- 

 cerned. It took seventy seconds to complete the process. One 

 of the antennae that had been dragging before the molt occurred 

 was dropped off with the exuvia. Immediately after the molt 

 the fore legs were noticeably smaller than usual. During the 

 process the isopod stayed in one place and disregarded all cur- 

 rents. A current set up immediately after the molt was com- 

 pleted was also disregarded. After about two minutes in the 

 same place the isopod moved across the pan and stopped in 

 an angle 17 cm. away. It rested there quietly for seventeen 

 minutes when it was again tested ten times for rheotactic reac- 

 tion. No movement occurred. At thirty minutes after molting 

 ten trials showed 30 per cent positive, 60 per cent indefinite 

 and 10 per cent no response, with an efficiency of 2.4. 



At this time the isopod was much more sensitive to touch 

 stimuli than usual. In the response just given it twice ran into 

 another isopod with its antennae and jumped back over a centi- 

 meter each time, although usually there would have been almost 

 no reaction. 



Three hours after molting the response was 70 per cent posi- 

 tive, 10 per cent indefinite and 20 per cent no reaction. The 



