GEOTROPISM OF PARAMECIUM AND SPIROSTOMUM . 5 



Harper also claims that the posterior end of normal paramecia 

 is heavier, because they assume the same position in respect to 

 gravity as do those paramecia which have ingested iron in their 

 posterior ends, and which are therefore undoubtedly heavier. 

 But a generalization from such an experiment is not always safe. 

 Spiders, for instance, whose posterior region or abdomen is 

 unmistakably larger and heavier than the anterior region, 

 always orient the head downward at rest} Through his experi- 

 ments, therefore, Harper is hardly justified in drawing his 

 conclusion. Again, according to him, there is no negative 

 geotropism of an active kind on the part of the normal para- 

 mecia, though Lyon thinks there is, because the "pull of gravity' 

 "is too weak to stimulate," unless it is augmented by "a greater 

 force" like a strong centrifugal force. "When, however, a 

 stronger force is substituted for gravity so as to produce a sudden 

 orientation, the irritability is affected, and the animal reacts to 

 the change" (3, p. 999). 



This conclusion was based on the rheotropism of paramecia. 

 'When . . . the centrifugal effect," he says, "exceeds the pull 

 of gravity and produces too sudden an orientating tendency, this 

 may act as in the ordinary rheotropic reaction against a current. 

 When strongly centrifuged the animal takes a position so that 

 it moves in the water just as the water moves past it in the 

 rheotropic response." "If it allowed itself to be oriented by the 

 centrifugal force with posterior end in advance its relation to 

 the water would be the reverse of what it is in the rheotropic 

 response" (3, p. 998). 



From this point of view Harper criticizes Lyon's results. 

 According to the former, "the inference that the anterior end is 

 heavier is contrary to what the shape of the body would indicate, 

 unless the heavier particles are located anteriorly." So he 

 suggests "as an explanation of Lyon's experiment that in strong 

 centrifugalization the same effect is produced at the outset as 

 by mechanical agitation, i. e., the reaction changes to positive." 

 After a repetition of Lyon's centrifugalization experiments, 



1 Moreover, the larvae of a marine annelid, Arenicola, the anterior end of which 

 is larger and heavier than the posterior end, are negatively geotropic. The writer 

 has found by centrifuging that its anterior end is heavier than the posterior end, 

 and further experiments are still in progress (1913). 



