PHYSIOLOGY OF THE INVERTEBRATA. 305 



co-ordinating centre between nerves pi'oceeding from it in 

 four directions — viz., to the attached tentacle, to the margin 

 on either side, and to the manubrium." 



" The nervous connections between the tentacles and the 

 manubrium are of a more general character than those between 

 the tentacles themselves ; that is to say, severing the main 

 radial nerve-trunks produces no appreciable effect upon the 

 sympathy between the tentacles and the manubrium. 



"The nervous connections between the whole excitable 

 surface of the nectocalyx and the manubrium are likewise of 

 this general character, so that, whether or not the radial 

 nerve-trunks are divided, the manubrium will respond to 

 irritation applied anywhere over the internal surface of the 

 nectocalyx. The manubrium, however, shows itself more 

 sensitive to stimuli applied at some parts of this surface than 

 it is to stimuli applied at other parts, although in different 

 specimens there is no constancy as to the position occupied 

 by these excitable tracts." 



Eomanes has examined the distribution of nerves in 

 Tiaropsis (especially T. iifidicans*), Staurophora, Aurelia, and 

 other Medusce. In all these forms primitive nerves are well 

 developed. By the word " nerves " is meant certain physio- 

 logically differentiated tracts of tissue, which either stimu- 

 lation or section prove to perform the function of conveying 

 impressions to a distance. 



Eomanes has also studied the subjects of co-ordination 

 and natural and artificial rhythm in the Medusce; but it is 

 not our object to detail these important investigations, as a 

 Tall account of them will be found in the Philosophical Trans- 

 actions of the Royal Society, to which our readers are referred. 

 Nevertheless, the following may be taken as a general 

 summary of the results : — 



(i) That in the covered-eyed Medusce the lithocysts are 



* This species was first described by Romanes ; see Journ. Linn. Soc. 

 (Zool.), vol". 12, p. 524. 



U 



