3o8 PHYSIOLOGY OF THE INVERTEBRATA. 



have been shown to exist between the tentacula, but not 

 between the tentacula and polypite. Section of the neuro- 

 muscular sheet proves that in the case of this genus physio- 

 logical harmony may, as a rule, be easily destroyed, although 

 it occasionally happens that such is not the case. 



(ii) Romanes has shown that the essentially nervous 

 function of maintaining excitational continuity is able to 

 persist in these primitive nervous tissues after they have 

 been subjected to the severest forms of section. This fact 

 " cannot be explained by Kleinenberg's theory of double- 

 function cells ; for sometimes contractile waves will become 

 blocked by section before the tentacular waves, and sometimes 

 vice versd. We seem, therefore, driven upon the theory of a 

 nerve-plexus, whose constituent elements are capable of 

 vicarious action in almost any degree." 



(12) Contractile waves in Aitrelia travel at the rate of 

 18 inches per second, if the temperature of the water is 

 normal; but the rate is greatly modified by temperature, 

 straining, anaesthetics, and various foreign substances. Stim- 

 ulus-waves only travel at the rate of 9 inches per second, if 

 the stimulus which starts such a wave is not strong enough at 

 the same time to start a contractile wave ; but if the stimulus 

 is strong enough to start both waves, they both travel at 

 about the same rate. 



(13) There appears to be no further co-ordination among 

 the lithocysts of the covered- eyed Medusce than such as arises 

 from contractile waves coursing rapidly from one of the 

 number, and, as it passes the others, causing them succes- 

 sively to discharge ; but, in the case of the naked-eyed 

 Medusce, true co-ordination has been proved to occur between 

 the marginal ganglia, and the tracts through which it is 

 effected have been proved to be the marginal nerves. Slightly 

 cutting the margin of a naked-eyed Medusa exerts a very 

 deleterious influence upon the vigour of the animal; and 

 violent nervous shock, while it always suspends both 

 spontaneity and initability, will sometimes also destroy 



