CONTROL BY THE ORGANISM 123 



activation of these cilia by nerve stimulation, they have not found 

 inhibition; in fact, Lucas found that the cilia are normally 

 quiescent, but are activated by stimuli from nerves of para- 

 sympathetic origin. Lucas and Loren (1935) could find no change 

 in the ciliary activity of turtle tracheal epithelium when the nerves 

 supplying the trachea were stimulated. 



A nemertine that moves by ciliary action was found by Friedrich 

 (1933) to show a reversal response rather like that of Paramecium. 

 If the head is touched while the animal is moving forwards, the 

 cilia stop and then reverse to move the animal backwards. The 

 cilia remain motionless after decapitation but, if the cut end is 

 stimulated, the cilia will start to beat in reverse. It is difficult to 

 determine whether this is a combination of inhibitory and 

 excitatory control, or whether we ought to make an additional 

 class for control which involves the reversal of ciliary beat, as 

 suggested by Fedele (1926) after he had studied a number of 

 metazoans. 



Other important examples of inhibition of locomotory cilia 

 should be mentioned. The planktonic veliger larvae of molluscs 

 are moved by the fringe of large compound cilia on the velum. 

 Carter (1926) found that these cilia do not beat continuously, 

 but that they show periodic intermissions and may also stop if the 

 animal is stimulated, so that it appears as if the ciliary beat is 

 under the control of the animal. The cilia of isolated cells beat 

 continuously without intermissions, and, in the presence of such 

 narcotic drugs as nicotine or morphine, the intermissions of 

 normal velar cilia ceased at concentrations which were too low to 

 have any other obvious effect on the beating activity. The in- 

 hibition appears to be caused by the action of nerves which run 

 around the velum from the cerebral ganglia and give off nerve 

 fibrils whose endings are intracellular and near the basal bodies of 

 the cilia (Carter, 1928). The ctenophore Beroe is moved by the 

 action of comb -plates which Gothin (1929) found to be controlled 

 by inhibitory impulses from the nervous system of the animal. 

 Chloral hydrate (0-1 to -2 per cent) did not affect the automicity 

 of the ciliary beat, but prevented inhibition. Inhibitory control 

 from the nervous system is also said to control the locomotion of 

 cilia of some of the smaller snails, e.g. Alectrion (Copeland, 1919), 

 and the cilia of the lips of Physa (Merton, 1923). 



