702 MEDUSiE OF THE WORLD. 



NaCI in the absence of the other salts of sea-water at first stimulates and later retards 

 pulsation, and this eifect is common!}- reversible. According to Bethe, the absence of calcium 

 ill the presence of the other salts of sea-water causes all movement to cease but normal 

 pulsation is restored by restoring the calcium to its normal amount. In a slight excess 

 in from 2 to 5 c.c. of J molecular CaClz in 100 sea-water calcium operates for a long time 

 to accelerate and strengthen pulsation. In larger excess such as 10 CaCU in 100 sea-water 

 it retards pulsation and produces an abnormal duration of s\stole. Magnesium chloride 

 and sulphate produce a marked primar\- retardation, without an\- final acceleration. Potas- 

 sium tends primarily to stimulate the rhythmical movement in Rhizostoma. Thus, accord- 

 ing to Bethe, Na, K, and Ca of the sea-water are primarily stimulants for pulsation, but are 

 antagonized bv Mg which alone is an inhibitor of pulsation, and thus a balanced solution is 

 formed. Na and Mg exert their effect primarih through the marginal sense-organs, and Mg 

 also acts upon the general s^•stem ot muscles. Na, on the other hand, affects the nervous sys- 

 tem. Potassium exerts its influence through the marginal sense-organs but has little or no 

 direct influence upon the muscles and nervous network. Calcium in slight excess exerts all 

 or nearh' all of its effect through the marginal sense-organs. 



.Aluminium is an inhibitor of pulsation and is relativeh' more powerful in this respect than 

 IS magnesium. MgSO, is a less powerful inhibitor than MgCl, and the order of efficienc\' as 

 stimulants of the sodium salts is beginning with the most powerful Na2 SO^, NaCl, Nal, 

 NaBr, NaNO,, and of the potassium salts K,SO,, KCl, KNO3. The rhythmical move- 

 ment of the whole medusa is controlled bv the marginal sense-organs. 



The action of the cations of the sea-water upon Rhizostoma appear, from Bethe's work, to 

 be similar to their effect upon Cassiopea, and I believe that the two medusae are controlled in 

 the same manner. (See Cassiopea xamachana.) 



I think, therefore, that Bethe is mistaken in concluding that potassium and calcium 

 stimulate pulsation. It is true in both Rhizostoma and Cassiopea that pulsation endures longer 

 in NaCl + KCl or in NaCl +CaCl, than in pure NaCl, but its rate is slower. Indeed the rate 

 of pulsation in NaCl is so abnormally rapid that the medusa soon comes to rest exhausted, and 

 this exhaustion is partially prevented by the subduing efl^ects of K or Ca and thus the pulsation 

 endures longer but at a slower rate. The apparent stimulation upon adding calcium in excess 

 is nor due to the direct effect of the Ca, but to the fact that Ca counteracts the stupef\ing 

 effects of Mg, thus permitting the sodium to act more effectivel}' as a stimulant. Thus the 

 sea-water is a balanced fluid, the stimulating effect of the Na cation being offset by the stupefy- 

 ing efl^ects of Mg, K, and Ca. 



Hargitt. 1904, has studied regeneration in R. pulmo. and he finds that it possesses a 

 moderate capacity, in confinement, for replacing lost rhopalia. Often 2 rhopalia regenerate 

 in the place of the one which has been cut away. The medusa has but little power to regenerate 

 mouth-arms or gastric lobes. 



Hesse, 1895, has made an elaborate study of the histology of the marginal sense-organs 

 and of the nervous network of the subumbrella. Definite strands of nerve-fibers extend 

 radially inward from the marginal sense-organs under the subumbrella epithelium. These 

 radial-fibers bend circumferentiallv at the zone of the ring-canal, forming a wide ring-like 

 band of nerve-fibers on the inner side of (centripetal to) the ring-canal. These nerve-fibers 

 arise from bipolar ganglia which are found in considerable numbers in the ectoderm along 

 the lines of the radial-canals which extend to the sense-organs. The bipolar ganglia are 

 derived from ectodermal cells of the subumbrella. 



The ectodermal cells of the marginal sense-organs bear cilia, and some of them are 

 sensory while others are supporting cells. The sensory pit on the exumbiella side of the 

 sense-club is also lined with a ciliated, ectodermal epitheliuin containing sensor\- and support- 

 ing cells, and also ganglion cells which send nuinerous nerve-fibers downward through the 

 gelatinous substance into the entodermal core of the sense-club, where thev form a felting 

 of fibers extending into the region of the entodermal concretions. 



The inner sensory pit on the subumbrella side of the sense-club is also lined with an 

 epithelium similar to that of the exumbrella pit, and it also sends nerve-fibers into the sense- 

 club. The sensory and ganglion cells of these 2 pits and of the sense-club constitute the 

 central nervous system of the medusa. 



