18 THE USES AND ORIGIN 



met with. The recent investigations of G. J. Romanes* 

 in regard to this subject are particularly interesting, 

 because they seem to show such a system actually in pro- 

 cess of evolution. The contractions of the bell-shaped 

 swimming disc of common Medusae must be familiar to 

 most dwellers by the seaside, and we now learn that this 

 part is lined internally by a very thin layer of highly 

 contractile protoplasm, not yet presenting the definite 

 characters of muscle. We learn also that this contractile 

 layer is permeated by a network of incipient nerve fibres, 

 in connection with rudimentary ganglia, near its free 

 margin. The degree of irritability of these altogether 

 elementary animal tissues, and the rate at which stimuli 

 traverse them, is alike remarkable, and far ahead of what 

 may be met with in the plants in which analogous 

 changes are most marked, — such as the Venus fly-trap 

 or the Sun- dew. 



According to Romanes the molecular discharges issuing 

 from a single rudimentary ganglion, in the swimming bell 

 of a large Aurelia weighing thirty pounds, were sufficient 

 to incite vigorous contractions throughout the whole mass 

 — though this mass weighed 30,000,000 times as much 

 as the ganglion itself. When all the ganglia have been 

 removed, he has found that a wave of contraction, starting 

 from any part of the disc which is touched, will travel 

 equally in all directions at the rate of a foot and a half 

 per second, so that the contraction of the whole bell is 

 practically simultaneous — and therefore, in marked con- 

 trast with the very slow bending of the irritated tentacle 

 of a Sun- dew. 



Thus the preliminary conditions already asserted to be 

 necessary for the initiation of a nervous system are here 

 present to a well-marked degree, and in notable contrast 

 * "Phil. Trans.," Part I., 1876. 



