I3O MANTON COPELAND. 



mud snail made microscopic observation of the cilia on the 

 ventral surface more difficult, but they could be seen when 

 beating, and those along the anterior border were clearly visible 

 at all .times. 



A comparison of the habits of the two species brought out one 

 striking difference. Whereas Alectrion trivittata often floats or 

 moves upside down at the surface of the water after the manner 

 of certain fresh-water gastropods, Alectrion obsolete, was never 

 observed to do so. Dimon ('05) in her monograph on the latter 

 species states that young individuals may float on the surface 

 but that the older ones very rarely do, and then only under 

 unusual circumstances, and concludes that the shell of the adult 

 is too heavy to permit it. It was impossible therefore to ob- 

 serve the behavior of the cilia on the foot of a freely floating mud 

 snail. 



The study of the two species of Alectrion shows unquestionably 

 that beating cilia are characteristic of a moving snail, and that 

 when the animal is at rest the cilia are motionless or exhibit 

 only local activity. It is equally clear that the rate of ciliary 

 movement is correlated with that of locomotion. 



The reason why Parker, in the investigation referred to, 

 failed to find cilia on the foot of Alectrion obsoleta was undoubt- 

 ably because when he placed carmine on them they happened to 

 be quiescent. Only the unusual behavior of the cilia prevented 

 their immediate discovery by the test he applied. 



COMPARISON OF LOCOMOTOR WITH CILIARY RATES. 



The foregoing observations are strongly in favor of the view 

 that progression in Alectrion is accomplished by ciliary action. 

 In order to test these conclusions further, the rate of locomotion 

 was compared with the rate at which grains of carmine were 

 driven over the surface of the foot by the beating cilia. The 

 latter may be termed the ciliary rate. By the use of a stop- 

 w r atch it was an easy matter to secure a series of records indicating 

 the speed at which the snails traveled a distance of 5 mm. over 

 the bottom of a glass dish containing sea water. In order to 

 obtain a ciliary rate, the snail was fixed upside down on a support 

 so that its foot when expanded was at the surface of the water. 



