032 MEDUS.E OB' THE WOHLI). 



lappets and rhopalia are in all respects similar to those of the Semaeostomeae. The muscular 

 svstem of the subumhrella is wiii-dtveiopt'cl and these forms are usually vigorous swimmers, 

 although in Ctissiopea we find that the medusx commonly remain upon the bottom with their 

 oral sides uppermost, and the pulsations of the umhrcila serve mainly to stir up currents which 

 ma\- bring food to the mouths. 



At the center of the subumbrella we find ;i thick, disk-shaped, gelatinous projection called 

 the arm-disk, for the 8 adradial mouth-arms arise from its lower side. This arm-disk is 

 merely the lower wall of the stomach wiiich has become thickened in order to give support 

 to the heavy gelatinous mouth-arms. In ail forms, however, having a unitary, cruciform, 

 genital cavity, an open space lies between the arm-disk and the stomach so that the arm- 

 disk is suspended from the subumjireila bv 4 thick perradial columns which are separated 

 one from another by the 4-ra)'ed genital porticus, which opens to the outer world by 4 inter- 

 radial ostia which alternate with the columns. 



The cruciform, central stomach dips downward into these perradial colunms and 4 

 bifurcated or 8 simple canals arise from the stomach and extend dow-nward into the 8 

 adradial mouth-arms, giving off numerous branches to the mouths. The \b canals to the 

 scapulets, when these are present, arise from these 8 mouth-arm ducts, as do also the canals 

 to the arm-disk, which fuse into 4 and finally into a single, central duct at center of arm-disk. 

 The central stomach also gives rise to canals which radiate outward through the subum- 

 brella of the bell. These may be connected by one or more ring-canals, or by networks of 

 anastomosing vessels. 



The facility with which some of these medusae may be maintained alive in aquaria has 

 permitted certain physiological work to be performed upon them. Bethe, 1903, 08, 09, studied 

 the rhythmical pulsation of Cotylorhiza and Rliizostoma, and Mayer, 1906, 08, carried out 

 experiments upon Cassio/>i-fi. Bethe finds that the pulsation resembles that of the vene- 

 brate hean in all imponant respeas. The pulsation-stimulus is nervous in nature, and the 

 "all or none" principle applies to medusae as it does to the vertebrate heart, as does also the 

 phenomenon of the refractory stage of Marey, 1876. A definite interval of time elapses between 

 the passage of the nervous stimulus and the response of the muscles, and the pulsation is a 

 reflex due to a constantly present stimulus, the response to which is periodic, because alter 

 the nerves have responded to the stimulus the>- become incapable ot reacting to it until after a 

 definite interval of rest, this resting period being called the refractory stage. 



Bethe, 1908, 09, in his stud\- of Rhizostoma ptihuo comes to conclusions in respect to the 

 effects of the ions of sea-water upon pulsation, which are in accord with those of Mayer. 1906. 

 (See Rhizostoma pulmo.) 



Mayer, 1906, 1908, working upon Cassiopea, found that the sea-water is a balanced fluid, 

 neither stimulating nor inhibiting pulsation. This is due to the fact that the stimulating 

 efl^ea of the sodium ion of sea-water is counterbalanced h\ the inhibiting influences of the 

 calcium, potassium, and magnesium. The stimulus which produces pulsation is due to the 

 constant maintenance of a slight excess of the sodium cation in the marginal sense-clubs, over 

 and above its concentration in the surrounding sea-water. This excess of sodium is main- 

 tained by the constant production of sodium oxalate in the terminal entoderm of the sense- 

 cluhs. This oxalate precipitates calcium to form the calcic oxalate crystals of the sense-club 

 and sets free sodium chloride the sodium ion of which acts as a nervous stimulant. Details 

 of these researches upon pulsation are given in the accounts of Cotylorhiza tuberculata, 

 Rhizostoma piilmo, and Casstopca xamachana. 



Hargitt, Zelen\', and Stockard have studied regeneration in Rhizostomir. Zeleny stated that 

 in Cassiopea the greater the number of arms removed up to 6 the more rapidl\' does each and 

 every arm regenerate, but this is refuted by Stockard, who further shows that the regenerating 

 tissue has a greater ability to absorb nutriment than have the normal, somatic body tissues, 

 and that in consequence of this the hod\- shrinks in size in direct proportion to the growth of 

 the regenerating arms, the growing arms reducing the bod\- as do cancer cells in their prolifer- 

 ation. Stockard also shows that cuts near the center regenerate more rapidl\' than those near 

 the margin of the disk, this being in accord with Morgan's law that the deeper the level of the 

 cut the more rapid the rate of regeneration. In Rhizostoma pulmo Hargitt found that two 

 rhopalia sometimes regenerate in the place of one which he had removed, and I have observed 



