230 Universifij of California Puhlications in Zoology [Vol. 11 



8. Reactions to Gravity 



In collecting material for this work, it was observed that the 

 most successful place for dredging for the Dinas was on the 

 bottom of the pond. When brought into the laboratory, and 

 placed in aquaria, the leeches crawl around on the bottom of the 

 jar or rest quietly in masses underneath stones. It is rather 

 rare that a specimen of Dina microstoma is to be seen on the 

 sides of the aquarium ; however, this is not true with GlossipJwnia, 

 since when the influence of light is removed these forms fre- 

 quently collect in groups along the sides of the jar. When a 

 Glossiphonia is placed on a vertical glass plate, however, it will 

 usually incline downwards and crawl to the bottom of the dish. 

 A Dina placed in a glass tube bent at right angles, and sub- 

 merged in the water, will in the majority of eases come to rest 

 in the horizontal portion. These observations lead one to the 

 conclusion that this species is normally positively geotactic. 

 Under certain conditions, there seems to be a tendency towards 

 the reversal of this reaction. In his work on the leeches of 

 Japan, Whitman (1886) says: "I have never seen the land 

 leech of Japan on trees, and I believe it keeps itself habitually 

 on the ground, or in the moss, or under damp leaves and loose 

 rubbish. When awakened by the footsteps of man or beast it 

 quickly appears on the surface, and frequently ascends low 

 plants and occasionally perhaps trees in search of the intruder. ' ' 



When stimulating a specimen of Dina microstoma to induce 

 fatigue it was observed that there was manifested more than a 

 dozen times the attempt on the part of the animal to crawl out 

 of the dish. This suggested that what appeared to be a reversed 

 geotaxis might be a tendency for the animal to crawl out of an 

 environment unfavorable to it. So jars filled with, and inverted 

 in, water boiled for thirty minutes and cooled in a corked flask 

 were supplied with a number of leeches. Increase of undulatory 

 movements was observed, but the reduction of o.xygen .seemed to 

 have no efl'ect on the geotaxis of the animal. 



Not so with mechanical shocks, such as jarring. Dozens of 

 times it was noticed in the aquaria on my laboratory table that 

 when these were shaken by a sudden jolting of the table large 



