BEHAVIOR IN LOWER METAZOA 



255 



Convoluta is found, and the waves would wash the animals away, if 

 their behavior did not prevent. As the water rises and the waves begin 

 to beat on the sand near them, they go downward into the sand, where 

 they are protected. As the water sinks, the animals creep upward and 

 appear again at the surface. These upward and downward movements 

 are reactions with reference to gravity, as is shown by placing the animals 

 on smooth, inclined, or perpendicular surfaces. They go downward 

 as the tide rises, upward as it falls. Bohn (1905) has shown that many 

 littoral mollusks and annelids show similar movements with relation to 

 the tides. 



The peculiarly interesting fact concerning this behavior in Convoluta 

 is the following: This periodical alternation of reactions, produced by 

 an environmental factor, becomes so impressed on the organization of 

 the animal that it occurs even when this factor is lacking. The alterna- 

 tion of movement has become habitual. If the worms are removed 

 to an aquarium where the tide no longer acts upon them, they continue 

 to go downward at the period of high tide, upward at the period of low 

 tide. This continues for about two weeks, so that the worms may be 

 carried far away from the shore, and may then be used for a time as tide 

 indicators. But under such conditions the periodicity after a time dis- 

 appears, showing that it was really due to the external factor, — the tides. 

 This appears to be the lowest known case of what we call in higher 

 animals a habit. 



In some of the higher invertebrates, lasting modifications of behavior 

 of a still more complex character may be induced experimentally. This 

 has been accomplished in the Crustacea by Yerkes (1902), Yerkes and 

 Huggins (1903), and Spaulding (1904). 



With the crayfish and crab, Yerkes and Huggins (1903) studied 

 the modification of behavior in escaping from danger and in finding 

 water. The crayfish was placed in one 

 end of an inclined pen which opened at 

 the other end into the water. The pen 

 was partly divided by partitions in such a 

 way as to leave two passages leading to 

 the water (Fig. 139). Either of these pas- 

 sages could be closed at its end by a glass 

 plate G. The animal was placed at T 



(Fig. 139). In moving away from this After Yerkes. See text 



region it might enter the blind pocket at G, thus not directly reaching the 

 water, or it might go through the other passage straight to the water. 



After some preliminary experiments without closing either passage, 

 showing that the animals were as likely to pass to the right as to the left, 



Fig. 139. — Pen used by Yerkes 

 in experimenting with Crustacea. 



