142 THE ELEMENTARY NERVOUS SYSTEM 



(Parker, 1896). Torrey (1904 a), however, showed that 

 in Sagartia this reversal could be brought about by me- 

 chanical stimuli as well as by chemical means and that it 

 was favored by a starved condition of the animal. Alla- 

 bach (1905) also found that in Metridium a ciliary re- 

 versal could be induced by mechanical means, and Gee 

 (1913) has recently shown that specimens of Cribrina 

 which have been in the laboratory some time do not ex- 

 hibit a reversal to mechanical stimuli, whereas those still 

 in their native pools give evidence of it. 



Further investigations have shown the correctness of 

 Allabach's contention (1905) that in Metridium margin- 

 atum some individuals on mechanical stimulation reverse 

 their ciliary stroke readily, others 1 less readily, and still 

 others not at all, variations largely dependent upon 

 whether the animals have been starved or fed. Two un- 

 derfed specimens of Metridium which on being tested 

 were found to reverse their cilia to clean filter-paper were 

 vigorously overfed and after three hours were tested 

 again with bits of clean filter-paper. In both instances 

 the paper failed to bring about a reversal of the cilia and 

 consequently was rejected. In another test made eighteen 

 hours after feeding, the paper was engulfed, showing that 

 the cilia had returned to the state characteristic of ani- 

 mals that had lacked food. It is, therefore, clear that an 

 underfed Metridium will reverse the effective stroke of 

 its oral cilia to mechanical stimulation, though a small 

 supply of food will obliterate this peculiarity and leave 

 these organs incapable of such reversal. 



The occasion of this loss of the power to reverse the 

 stroke of the oral cilia on mechanical stimulation has 

 been ascribed by Allabach (1905) to the difference in 

 metabolism between a well-fed and an underfed indi- 



