1877.] 



Locomotor System of Medusa. 



483 



is a remarkable thing that the contractile tissues, although themselves 

 incapable of originating a spasm in response to irritation, are never- 

 theless so wonderfully capable of conducting a spasm when this has 

 been originated by irritation of the slender tissue tracts above named. 

 It is as though every fibre or cell of the general contractile tissues is 

 able to liberate energy in either of two very different ways ; and whenever 

 one part of the general mass is made to liberate its energy in one of 

 these two ways, all the other parts of the mass do the same, and 

 this no matter how far through the mass the liberating process may 

 have to extend. Or, to employ a somewhat far-fetched but convenient 

 metaphor, we may compare the general contractile tissues of this Medusa 

 to a mass of gun-cotton, whicli responds to ignition (direct stimula- 

 tion) by burning with a quiet flame, but to detonation (marginal stimu- 

 lation) with an explosion. Isow to say that it is the ganglionic element 

 of the margin or radial tubes which here acts as the detonator, is not 

 to explain the facts. Doubtless it would be an interesting thing to 

 know that a ganglion-cell may be able to originate two very different 

 kinds of impulse according as it liberates its energy spontaneously or 

 in answer to direct stimulation ; but this knowledge would merely serve 

 to transfer the questions which now apply to the marginal and radial 

 tube tissues in general to the ganglionic tissues in particular. Again, 

 the supposition of the ganglia acting as detonators when themselves 

 directly irritated, would in nowise tend to explain why it is that the con- 

 tractile tissues are capable of two such very different kinds of response. 



In conclusion, I may state that when a tonic spasm is being 

 slowly recovered from, one may often observe rhythmic locomotor con- 

 tractions superimposed on the general spasmodic contraction. Again, 

 anaesthetics block spasmodic waves ; but not till after they have sus- 

 pended spontaneity, and even destroyed muscular irritability as regards 

 direct stimulation. Up to this stage the certainty and vigour of the 

 spasm is not perceptibly impaired ; but soon after this stage the inten- 

 sity of the spasm begins to become less, and, later still, it assumes a local 

 character. It is important also to notice that at this stage of ansesthe- 

 siation the effect of marginal irritation is very often that of producing 

 a general locomotor contraction, and sometimes a series of two or three 

 such. During recovery in normal sea-water all these phases occur in 

 reverse order. 



§ 5. Bate of transmission of Stimuli in Aurelia aurita. (A) Contrac- 

 tile luaves. — The rate at which contractile waves traverse spiral strips of 

 Aurelia is variable. It is largely determined by the length and width of 

 the strip. In the nnmutilated animal the rate is from 18 to 20 iuches 

 per second in water at 40° to 45°. Temperature exerts a wonderful 

 influence on the rate, as the following example will show : — 



