SCYPHOMKDUS^. 



503 



Romanes and Eimer found that if the marginal sense-clubs of Scyphomedusas be removed 

 a more or less complete paral}sis results, although the disk remains capable of responding 

 temporarily to any stimulus. Bethe, 1903, found that in Rhizostoma pulmo and Cotylorluza 

 tuberculata the pulsation-stimulus is nervous in nature, and there are many analogies between 

 the rhythmic movement of these medusae and that of the vertebrate heart. For example the 

 "all or none" principle applies to medusze, as does also the phenomenon of the insensibility 

 of the medusa to stimulation while in systole, and the extra systole and compensating period 

 of rest. Mayer, working upon Cassiopia xamachana, finds that the sea-water is a balanced 

 fluid, the stimulating tendency of its sodium being exactly counterbalanced by the inhibiting 

 effects of its magnesium, calcium, and potassium. The pulsation stimulus arises in the mar- 

 ginal sense-clubs and is due to the constant formation of sodium oxalate in the entodermal 

 cells at the distal end of each club. This precipitates the calcium chlorides and sulphates 

 of the sea-water forming the insoluble calcic oxalate crystals of the sense-club, and setting 

 free sodium chloride and sulphate the cations of which are powerful nervous stimulants, and 

 produce the periodic response of the nervous elements which causes the rhythmic pulsation. 



In the h3dromedusa£ on the other hand the function of the control of pulsation is not 

 localized in the sense-organs for any part of the bell-rim will serve to maintain the bell in 

 pulsation. 



It is interesting to note that whatever the effect of the several cations, sodium, magnesium, 

 calcium, and potassium, may be upon the neuromuscular system, their effects upon the move- 

 ment of cilia is the exact opposite. Whatever stimulates muscles inhibits cilia, and vice versa. 



The gastric tentacles, which arise from the subumbrella wall of the stomach on the sides 

 of the interradial septa are very characteristic and constant structures in Sc3phomeduss. 

 Thev are entodermal with solid cores of mesogloea, and are in no sense comparable with the 

 marginal tentacles, but their function is unknown. 



The marginal tentacles themselves are to be regarded as structures of the subumbrella. 

 They are usually but not invariably hollow and consist of an axial core of entodermal cells 

 encased outwardly by ectoderm which is richlv besprinkled with nematocysts. The marginal 

 sense-clubs are highly specialized tentacles which have been transformed into sensory centers. 

 Ocelli may or may not be present and when found they may be of ectodermal or of entodermal 

 structure; but concretions of crystalline nature are invariably found in the entodermal cells 

 at the distal end of the club in all forms exhibiting rhythmical pulsation. The gonads are 

 follicular foldings of the entodermal subumbrella floor of the stomach. 



