RHIZOSTOM.E — CASSIOPEA. 645 



Usually the 4 penadial tentacles are soon supplemented hy the 4 intcrradial ones, and in 

 about 3 days alter they Hrst appear the 8 tentacles are as long as the proboscis of the scypho- 

 stoma. 8 adradial tentacles then develop. The number and arrangement ot" the tentacles is, 

 however, very variable but finally there are about 32. In any case there are twice as many 

 tentacles as there are to be rhopalia. The full-grown tentacles are tapering, slender, and 

 about 3 times as long as the body of the scyphostoma. Half of them are erect, and the alternate 

 half stretch more horizontally outward. 



The 4 primary, gastric pouches are not formed by evagination from ectoderm and ento- 

 derm alternately, as in the sexually produced scyphostoma of /:?Mr£7/;(7, etc., according to Goette, 

 but are wholly entodermal as Hadii finds them to be in Chrysaora, and simply separated one 

 from another by the ingrowth of the 4 interradial septa. At first the septa are simple, entire 

 buttresses of entoderm with an axial sheet of mesogloea, but later each septum becomes perfor- 

 ated immediately under the interradial tentacles, thus forming a ring-sinus. There are 4 longi- 

 tudinal strands of septal muscles, i in the mesogloea of each septum. 



The septa bear no definite relation to the exact position of the interradial tentacles, for 

 these may arise on either side of or in the plane of a septum. In this respect the scypho- 

 stomae resemble those ot AureUia and Cotylorhiza according to Claus, and differ from the 

 Anthozoa, in which the tentacles are invariably interseptal. Every alternate tentacle stands 

 erect while the others extend outwardly. When the scyphostoma disk is about 2 mm. wide, 

 conical enlargements which contain crjstalline concretions are observed in the entoderm at the 

 bases of the erect tentacles. An ectodermal ocellus develops upon the aboral (lower) side 

 of each conical enlargement and the tentacle itself begins to degenerate, becoming finally 

 absorbed, leaving only the sense-club with its ectodermal ocellus and terminal mass of ento- 

 dermal concretions. When the tentacles begin to degenerate, slight pulsating movements of 

 the disk commence. The marginal lobes grow out while the rhopalia are being formed, and 

 finall}' the interrhopalial tentacles are also absorbed. 



Strobilization is monodiscus, but the scyphostoma after setting free the ephyra develops 

 new tentacles and gastric pouches, and may strobilate a second time. 



The \oung ephyra has the same number of marginal sense-organs as the adult medusa. 

 It has 4 simple lips and a central mouth-opening. Then the angles of the lips become extended 

 to form 8 oral arms, very much as in the adult Aurosa. Then there is a stage wherein the 

 oesophagus is divided into 4 tubes with 3 osculae and an oral vesicle on each arm. Rhizostoma 

 and Cotylorhiza go through a similar stage. The septal muscles and their funnel-cavities 

 disappear wholly in the ephyra, as do also the 4 interradial septa. 



Mayer, 1906, 1907, 1908, finds that the stimulus which produces pulsation in Cassiopea 

 is nervous in nature and will pass over newly regenerated tissue which contains nervous, 

 but no muscular elements. Moreover, if the muscles be paralyzed by magnesium the pulsation- 

 stimulus still travels through the nervous network of the subumbrella, even though the muscles 

 can not respond to its presence by contraction. 



If an annulus, or strip of any shape constituting a closed circuit, be cut from the sub- 

 umbrella and stimulated momentaril)' at any one point, 2 waves of contraction start in opposite 

 directions around the strip from this stimulated point. By pressing upon one side of the ring 

 we dampen and reduce the strength of the initial wave passing over that side, and when the 

 two waves meet the stronger wave overpowers and annuls the weak one. Thus a single 

 contraction-wave is entrapped in the ring-circuit and travels constantly around it at a uni- 

 form rate. The mechanical arrangement of the pulsating medusa in nature is such as 

 to prevent the formation of such continuous pulsation-waves — the pulsations are recurrent 

 and each contraction-wave is annulled as soon as it has produced a single contraction ot 

 the medusa. 



The sea-water is a balanced fluid for the medusa, neither stimulating nor inhibiting its 

 pulsations. This balance is due to the fact that the ionic sodium of sea-water is a powerful 

 nervous and muscular stimulant, but the magnesium, calcium, and potassium are inhibitors 

 and annul the stimulus produced by the sodium. \i calcium be absent the magnesium quickly 

 checks all pulsation. On the other hand, a slight increase in the sodium serves as a nervous 

 stimulus which overcomes the inhibiting tendency of the magnesium, calcium, and potassium 

 and produces contraction. 



