xlvi THE VOYAGE OF H.M.S. CHALLENGER. 



connected on the one side with the " sense epithelium " of the ectoderm lying below it 

 (namely, at the umbrella margin and the organs of sense), and on the other with the 

 underlying muscular plate (namely, at the subumbrella and the oesophagus). Inde- 

 pendent " ganglia " separated into units, or centralised nerve knots, and visible " nerve 

 fibres " consisting of bundles of separate nerve fibrdlae are only developed in a few sur- 

 faces {e.g., Charybdea, PL XXVI). We ought, however, to observe that the most recent 

 numerous and important researches on these difficult conditions are still too insufficient 

 to allow us to form exhaustive and certain conclusions on the subject. On the one 

 hand we know nothing of the nervous system of several principal groups of the Medusa? 

 (for example, of the two orders of Stauromedusse and Perornedusse) ; on the other hand, 

 in the remaining orders, the nervous system has not been examined on important 

 parts of the body, on which from their greater mobility and great sensibdity it is 

 probably very highly developed, pre-eminently on the oesophagus and the oral organs. 

 As far as we can judge at present the nervous system of the two sections presents 

 essential differences, as it appears more strongly centralised in the Craspedotae, more 

 diffuse in the Acraspedae. 



§ 73. Nervous system of the Craspedotae. In all Craspedotaa, of which the nervous 

 system has been minutely examined up to this time (and among these we find some 

 belonging to all the four orders), its important centre represents a double marginal 

 nerve ring, lying on the proper umbrella margin immediately outside the insertion of 

 the velum. It is covered externally by a ciliated sense epithelium, consisting of small 

 flagellate cells, and is divided by the supporting plate of the velum insertion into two 

 separate rings, an exumbral and a subumbral ring. The dorsal or exumbral nerve ring 

 (PI. IX. fig. 7, re' ; PI. XII. fig. 12, re') is the so-called upper ring (the outer or lower 

 in the normal position of the velum), and seems to be pre-eminently the central organ of 

 sense ; it contains smaller and scantier ganglion cells, also finer fibrillar, and specially 

 provides for the different organs of sense of the umbrella margin (namely, the auditory 

 clubs and the tentacles). The ventral or subumbral nerve ring (PI. IX. fig. 7, re" ; PI. 

 XII. fig. 12, re ) is the so-caUed " lower" ring (the inner or upper ring in the normal 

 position of the velum), and appears to be pre-eminently the motor central organ ; it 

 contains larger and more numerous ganglion cells, as well as several fibrdke, and provides 

 specially for the muscular system of the velum and the suburnbrella. The two nerve 

 rings are immediately connected by numerous fine filaments, which pierce the separating 

 fulcra! lamella of the insertion of the velum, and give out numerous filaments which 

 extend like a plexus and are in connection with many peripheric ganglion cells. In 

 many Craspedotas the nerve ring shows sbght swellings, which are perhaps radial 

 ganglia at the points of insertion of the tentacles (especially at the four perradial 

 and four interradial). 



§ 74. Nervous system of the Acraspedae. The structure of the nervous system in 



