THE HABIT-FORMATION IN THE GREEN CRAB. 243 



The only motive relied upon for the return of the crab to the 

 aquarium from the experimenting box was its desire to get into 

 the water. 



For two weeks several crabs were given on an average four 

 trials per day in the labryrinth. At the beginning of the work 

 they frequently wandered in the entrance passage and blind al- 

 leys several minutes before they succeeded in escaping, but after 

 twenty or thirty trials most of them were able to escape directly. 

 I was unable to get any satisfactory quantitative expression of 

 the rapidity of learning because of the great variation in strength 

 of the animal's desire to get back to the water ; at one time it 

 would move about constantly until it found the exit, at an- 

 other it would rest quietly in the passages for long periods. In 

 the case of an animal which, for a month, was given two trials 

 per day in the labyrinth, the escape at first took about five min- 

 utes, and at the end of the period about ten seconds. 



At first the crabs when placed in the box and poked with a 

 stick scurried off into a corner and took a defensive position; 

 whereas, after a few escapes from the box, they made directly 

 for the exit as soon as they were threatened with a stick. 



Even fifty experiences, in case of most of the individuals 

 studied, did not result in the formation of a perfect habit. 



Further tests of the ability of the green crab to learn were 

 made by placing a wire-screen partition in the middle of the 

 aquarium and allowing the animals to find their way through an 

 opening in the screen to food on the opposite side. Since the 

 crabs tend to follow the edge of such a screen, the opening, 10 

 cm. x 10, was made in the middle of the screen. It was there- 

 fore about 12 cm. distant from the sides, top and bottom of the 

 aquarium. 



In this test each crab was given a single experience per day. 

 To provide the animals with a motive for seeking to pass beyond 

 the screen, at the beginning of each trial a piece of fish meat was 

 placed in the corner of the aquarium diagonally opposite to that 

 from which the crab started. The meat having been placed, a 

 crab was put into the aquarium, and a record was kept of its 

 wanderings in search of the food, and of the time occupied in 

 finding it. 



