158 REPORT— 1876. 



On a Mieroscojpe adajited for sliowing the Circulation in tlic Human Subject. 



By Dr. Urban Pbitchard, 



Physiology of the Nervous System of Medvscf. 

 By George J. EojJAjfES, M.A., F.L.S., ^r. 



Fundamental Observations. — The autlior has succeeded in demonstrating tlie pre- 

 sence of a nervous system in Medusae, the ganglionic element of which appears to 

 be localized exclusively in the margin of the swimming-hell. For he found that 

 on excising the entire 'margin of the bell in any species of naked-eyed Medusse the 

 swimming motions of the bell instantly ceased and were never again resumed, 

 while the severed margin continued its rhvthmical contractions for days. With 

 the covered-eyed Medusse the case is not quite so definite ; for although the para- 

 lysis of the bell, which is here likewise produced by the operation just described, 

 is usually complete for a time, it is not always permanent ; but, after periods 

 varjing from a few seconds to half an hour or more, occasional contractions begin 

 to manifest themselves. Moreover, in the case of the covered- eyed Meduste, the 

 author found that excision of the lithocysts alone was attended with the same degree 

 of paralyzing effect on the bell as was excision of the entire margin ; whereas in 

 the case of the naked-eyed MedusaB such was not the case. Histological observa- 

 tions revealed the presence of ganglion-cells and nerve-fibres in the lithocysts. 



Natural Mh/thm. — As regards the natural rhythm of the Medusa;, it was ob- 

 served that its rate has a tendency to bear an inverse proportion to the size of the 

 individual; but that on submitting an individual to artificial segmentation, the 

 rate of the rhythm exhibited by the various segments showed a tendency, other 

 things equal, to vary directly as the size of the segment. 



When forms of mutilation were practised in which the margin of the swimming- 

 bell was left intact, it was observed that after a temporary acceleration the rate of 

 the rhythm progressively declined, and became stationary at a rate that was slower 

 the greater the amount of tissue that had been removed. From these experiments 

 the author is inclined to infer that the apparently automatic action on the part of 

 the marginal ganglia is really of the nature of a reflex — a constant stimulation being 

 presumably supplied b}^ those other parts of the organism the removal of which was 

 attended with a retardation of the rhj'thm. 



The rate of the rhythm is increased by elevations of temperature as far as 60° F., 

 but in still warmerwater (70°-S0°) the rate, after having been temporarily quickened, 

 becomes permanently slowed. Diminution of temperature likewise produces a 

 retarding efiect on the rhythm, and eventually (20°) altogether stops it. 



Some specimens of Aurelia aurita were frozen solid, so that all their gelatinous 

 tissues were pierced through in every direction by an innumerable multitude of 

 ice ciystals, which had been formed by the freezing, in situ,oi the sea- water which 

 enters so largely into the composition of these tissues. Yet, on being thawed out, 

 the animals recovered, although their original rate of rhythm did not fully return. 

 Their tissues then presented a ragged appearance, which was due to the disinte- 

 grating effect produced by the formation of the ice crystals. 



The rate of the rhythm is accelerated by oxygen and retarded by carbonic acid. 



Stimulation. — All the contractile tissues of all the Medufas are keenly sensitive 

 to all kinds of stimulation. When a swimmiDg-bell, for instance, is paralyzed by 

 excision of its margin, it invariably respouds to a single stiuuilus by once perforai- 

 ing that movement which it would have performed in response to that stimulus 

 had it still been in an uumutilated state. To mechnnical stimulation the sensitive- 

 ness of the paralyzed bells is wonderfully great — a drop of sea-water let fall from 

 an inch in height upon the contractile tissue being sutficient, in seme species, to 

 elicit a responsive contraction. In their responses to all kinds of chemical stimuli, 

 the excitable tissues of the Medusas conform in every respect to the rules v.'hich 

 are followed by the nervo-muscular tissues of higher animals. Similarly with 

 thermal and electrical stimulation. Light also acts as a powerful and unfailing 

 stimulus in the cases of some of the naked-eyed Medusje. Sarsia, for instance, 



