LOCOMOTION IN GASTROPODS. 129 



motionless. Moving cilia were observed slowing down and 

 coming to rest, and passive ones becoming slightly active. 

 Suddenly all the cilia under observation began to beat at high 

 speed, when compared with the slower movement first recorded, 

 and the foot process moved out of the field of vision. The 

 snail was now showing its normal surface "swimming" activity. 



The relation of ciliary movement to locomotion was finally 

 studied by observing under the microscope snails which were 

 moving on the under side of a glass slide placed across a dish full 

 of sea water. A resting animal showed the cilia on the anterior 

 border and ventral surface of the foot passive. Sometimes, 

 when the snail was feeding, one of the lateral foot processes was 

 moved or contracted slightly and some local beating of the cilia 

 of short duration occurred. When the animal was moving, all 

 the cilia which could be seen were active, and moreover, the 

 rate at which they beat was distinctly correlated with the speed 

 of locomotion. Rapidly beating cilia decreased their rate of 

 movement and came to rest when the animal did, and became 

 active again when locomotion began. Very satisfactory ob- 

 servations were made by placing the snails on the under surface 

 of a slide, which had been smeared with fish meat, and allowing 

 them to feed in this inverted position out of the water. Under 

 these conditions they frequently progressed short distances and 

 stopped, and the cilia in turn were seen to start beating, to 

 continue slowly while the animal moved, and then to come to 

 rest. When the rate of locomotion was increased, the cilia 

 beat so rapidly that they could only with difficulty be seen at 

 all. 



A microscopic examination of the cilia of snails moving and 

 floating at the surface of the water confirmed the observations 

 already reported. 



Alectrion obsoleta. The foot of Alectrion obsolete, resembles 

 that of Alectrion trivittata but is thicker, not so long in proportion 

 to its width, rounded at the posterior end and lacks posterior 

 tentacles. The behavior of the cilia were studied by some of 

 the methods described in connection with the w r ork on Alectrion 

 trivittata, and the results obtained were so closely similar it is 

 unnecessary to report them in detail. The thicker foot of the 



