503 



Romanes and Eimer found that it" the marginal sense-clubs of Scyphomedusz he removed 

 a more or less complete paralysis results, although the disk remains capable of responding 

 temporarily to any stimulus. Bethe, 1903, found that in Rhizostoma pidino and Cotylorhiza 

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

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

 "all or none" principle applies to medusa?, as does also the phenomenon ot the insensibility 

 of the medusa ro stimulation while in systole, and the extra M stole 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 entodennal 

 cells at the distal end ot 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 ot which are powerful nervous stimulants, and 

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



In the hydromedusac on the other hand the function of the control ot pulsation is not 

 localized in the sense-organs for any part of the bell-rim will serve to mainiain 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 ot cilia is the exact opposite. \\ hatever stimulates muscles inhibits cilia, and 



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 Scvphomcdus;e. 

 They are entodermal with solid cores of mesogloea, and are in no sense comparable \\ith the 

 marginal tentacles, but their function is unknown. 



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

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

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

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

 Ocelli may or mav nor be present and when found they may be ol ectodermal or ot entodermal 

 structure; but concretions ot crystalline nature are invariably (omul 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. 



