632 MEDUS.E OF THE WORLD. 



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

 system of the subumbrella is well-developed and these forms are usually vigorous swimmers, 

 although in Cassiopea we find that the medusae commonly remain upon the bottom with their 

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

 may bring food to the mouths. 



At the center of the subumhrella we find a 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 which has become thickened in order to give support 

 to the heavy gelatinous mouth-arms. In all 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 subumbrella by 4 thick perradial columns which are separated 

 one from another by the 4-rayed 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 columns and 4 

 bifurcated or 8 simple canals arise from the stomach and extend downward into the 8 

 adradial mouth-arms, giving off numerous branches to the mouths. The 16 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 medusas 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 Rhizostoma, and Mayer, 1906, 08, carried out 

 experiments upon Cassiopea. Bethe finds that the pulsation resembles that of the verte- 

 brate heart in all important respects. 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 after 

 the nerves have responded to the stimulus they become incapable of reacting to it until after a 

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



Bethe, 1908, 09, in his study of Rhizostoma pulmo 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 

 effect of the sodium ion of sea-water is counterbalanced by 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- 

 clubs. 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 pulmo, and Cassiopea xamachana. 



Hargitt, Zeleny, and Stockard have studied regeneration in Rluzostornie. Zeleny stated that 

 in Cassiopea the greater the number of arms removed up to 6 the more rapidly 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 body shrinks in size in direct proportion to the growth of 

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

 ation. Stockard also shows that cuts near the center regenerate more rapidly 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 



