Vol. 3] Bovard. — Condylostoma patens. 351 



but more important is the fact that food is more abundant on the 

 bottom. Gliding is always executed in the same manner, a struc- 

 tui'ally defined side being always next to the sub-stratum. Glid- 

 ing is thus set off from the movement in swimming, and cannot 

 be classed as a chance movement. 



Parts of the Body Concerned.- — During gliding the cilia all 

 over the body beat, but those of the dorsal surface less strongly 

 than Avhen swimming. When India ink is added to the water 

 containing the animals, the exact actions of the cilia are more 

 easily made out. In many cases the cilia on the dorsal surface 

 are seen to beat but slowly and those of the posterior tip at 

 irregular intervals. At times the cilia have a peculiar waving 

 motion, apparently beating first in one direction and then in 

 another ; on other occasions they do not beat at all. This is espe- 

 cially true of those individuals which keep the posterior tip 

 normally very close to the surface film. The body cilia, while 

 the principal factor in causing this gliding, are not the only 

 means, for it is certain that the membranellae play some part. 

 The action of this set of organs is as follows : the stout fan-like 

 membranellae by their vigorous backward stroke cause strong 

 currents to pass down the buccal groove and out on the left side, 

 through the arch formed by the raised portion of the left lip of 

 the adoral zone. Some idea of the force of this vortex that is 

 constantly passing posteriorly can be obtained by observing the 

 force with which particles pass in and out through the groove. 

 On one occasion one of these animals happened to strike another 

 and remained for a moment or two holding on to it by the suction 

 of the buccal groove. At the instant of release a distinct evagi- 

 nation of the cuticle of the attacked animal could be plainly seen. 

 The posterior end of the animal which plays such an important 

 part in the motor reaction is also important in governing the 

 direction of gliding. 



Position of the Body. — During this movement the anterior 

 end of the animal presents numerous changes. The whole adoral 

 zone is depressed so that the anterior margin of the buccal groove 

 comes close to the substratum. The right lip usually varies but 

 little, holding its place close to the bottom while the left lip may 

 03" may not be held in such close contact with the substratum. 



