THE ANALYSIS OF BEHAVIOUR IN ANIMALS 153 



among Coelenterates, a more specialised arrangement exists. 

 Nucleated cell-bodies intercalated in a reticulum of fibrils 

 connecting the sensory cells and muscular elements are con- 

 tinuous with one another ; there is between the sensory and 

 motor apparatus an uninterrupted network. 



Ingenious experiments on the physiological properties of 

 the nerve-net were performed by Romanes more than half 

 a century ago. Romanes worked on Aurelia, the common 

 jelly-fish of our coast. From the sensory tentaculocysts 

 the nerve-net extends inwards over the circular sheet of 

 muscular tissue round the mouth by whose contraction the 

 rhythmical pulsation of the swimming bell is brought about. 

 Excision of all the tentaculocysts brings about a cessation of 

 the pulsations. If, however, one marginal sense-organ is 

 left behind it induces a double wave of contraction — one to the 

 right and one to the left — and the rhythmical movements are 

 preserved. To ascertain whether special paths of conduction 

 exist in the nerve-net, Romanes made a series of incisions 

 in the bell. Spiral, circular, and interdigitating incisions in 

 the under surface of the body did not prevent pulsation so 

 long as at any point the nerve-net of the parts separated was 

 left in continuity, although the muscular coat might be com- 

 pletely severed. Thus one part of the nerve net is as good 

 as any other for the transmission of nervous impulses. The 

 same, according to Parker, is true of transmission in the trunk 

 region of the sea-anemone. But here definite paths of con- 

 duction seem to exist in the nerve-net in virtue of the fact 

 that the fibrils run pre-eminently in an oral-aboral direction. 

 Hence if the tip of a tongue of tissue cut from the wall of the 

 body in a longitudinal direction is stimulated, generalised 

 muscular contraction ensues, while if the tip of a tongue 

 of tissue cut in the equatorial plane is stimulated, no general 

 response is evoked. Such polarisation suggests how the 

 separation of a C.N. S. may in the first place have been brought 

 about. But the central nervous system of ccelomate animals 

 is fundamentally different in that the nervous elements are 

 not structurally continuous. The experiments of Romanes, 

 which were independent of and contemporaneous with others 



