152 COMPARATIVE PHYSIOLOGY 



beat simultaneously but in regular succession one after the 

 other with reference to the axis along which the ciliary current 

 is maintained. It is this that gives the active ciliary epithelium 

 an appearance like a field of corn blown by the wind. The 

 metachronial rhythm of cilia does not depend upon mechanical 

 stimulation of one cell by the ciliary activity of its neighbour. 

 There is abundant evidence for this statement, but perhaps 

 the most convincing is provided by experiments of Kraft, 

 who showed that by cooling a zone of ciliated epithelium till 

 mechanical activity subsided, effects of mechanical and 

 thermal stimulation on one side of the quiescent zone were 

 transmitted to the other. There is no clear evidence of 

 nervous agencies at work in connection with ciliary 

 mechanisms ; and it seems fairly certain that in general the 

 disturbance which underlies the metachronial rhythm is 

 propagated through the undifferentiated protoplasm of the 

 cells. Chambers has shown experimentally that in cells 

 with intercellular protoplasmic bridges the effects of an 

 injurious stimulus are transmitted from one cell to another. 

 Where there are no demonstrable protoplasmic connexions 

 this is not the case. Such transmission is usually called 

 neuroid, although there is as yet no evidence of its depending 

 upon the same type of mechanism as true nervous conduction. 

 The Elementary Nervous System.— The simplest possible 

 form of neuromuscular system is met with in the tentacles 

 of some sea- anemones {e.g. Cerianthus). Here the entire 

 complex consists of a sensory cell ending in a process 

 which arborises round the underlying muscle fibres. The 

 single cell combines all the functions of receptor, afferent, and 

 efferent neurone. The internal processes tend to run towards 

 the base of the tentacle, and with this, according to Parker, 

 is correlated a polarity in the response to stimulating the 

 tentacle at its distal and basal extremities respectively. To 

 quote from Parker, " when the tip of a tentacle is vigorously 

 stimulated the whole tentacle is likely to respond, but when a 

 part lower down in the side of the tentacle is stimulated, the 

 reaction is chiefly from this point proximally." 



However, in the trunk region of Actinozoa and in general 



