646 MEDUS.E OF THE WORLD. 



The pulsation-stimulus is engendered in the marginal sense-organs. A uric oxalate of 

 sodium is developed constantly in the entodermal cells of the outer end of each sense-club. 

 This oxalate precipitates calcium, thus forming the crystalline concretions which consist of 

 calcium uric oxalate, and at the same time it sets free such soluble stimulants as NaCl and 

 Na,So r Thus we find that the sense-clubs are engaged in the maintenance of a slight con- 

 centration of sodium over and above that found in the sea-water itself. This slight excess of 

 the sodium ion is a stimulant to the nervous elements within the sense-club and the nervous 

 elements respond to it recurrently, producing the rhythmical contractions of the muscles. 



If a disk without marginal sense-organs be set into pulsation and then disturbed by a 

 sudden current in the sea-water, etc., it displays excitement by markedly increasing the 

 amplitude of its pulsations. Hence its ability to display excitement is not dependent upon 

 the sense-organs, but upon the general nervous tissues of the subumbrella. 



When the marginal sense-organs regenerate, each one appears with a short, hernia-like 

 side branch, which disappears later. In this connection it is interesting to see that the sense- 

 organs are normally formed as side buds from the bases of each alternate tentacle of the 

 scyphostoma, and then the tentacles themselves are absorbed. Thus when they regenerate 

 they display a tendency to replace the tentacle as well as the sense-club. 



In 1909 I succeeded in grafting two individuals of C. xamachana, side by side, so that their 

 subumbrellas joined. The double-medusa then pulsated constantly at the rate of the faster 

 individual which initiated and controlled all of the rhythmical movements; but if one pinched 

 the controlled medusa its rate increased and it then assumed a temporary control of the 

 double animal. Hence the complex always pulsated at the rate of its fastest member. Hargitt 

 attained a similar result with 2 individuals of Gonionernus murbachii, but in this case the 

 rims were attached around nearly their entire edges so that any movement of one medusa 

 must necessarily cause a corresponding movement of the other. In the two Cassiopeas, 

 however, the contact was at a single narrow bridge of tissue only, and indeed the medusas pul- 

 sated independently until the nerve-nets of their subumbrellas joined in the process of 

 regeneration. 



The color of the umbrella of C. xamachana is mainly due to the presence of numerous 

 symbiotic algae, Tjooxanthella, which Bigelow finds contain starch, cellulose, and chlorophvl. 

 These plant cells are globular and occur in small clusters imbedded in the mesogloea and are 

 greenish-brown in color. 



A well-marked, conical, pit-like depression is occasionally seen upon the aboral side of 

 each mouth-arm near its point of origin from the arm-disk, but more commonly in male than 

 in female medusas. The female medusae greatly outnumber the males. Perkins believes that 

 the medusae may be hermaphroditic, but of this we have no evidence. Pseudorhiza haeckelii 

 is, however, known to be hermaphroditic, the spermaries being in the gutters of the mouth- 

 arms. 



Zeleny, 1907, finds that medusae maintained in pulsation appear to regenerate at about the 

 same rate as if the disk were at rest. Certainly the functional activity of pulsation seems to 

 be of no aid in accelerating regeneration, for Stockard also finds that the medusa regenerates 

 at practically the same rate whether it be pulsating or at rest. 



Stockard, 1907, discovered that tissues removed from various parts of the subumbrella 

 regenerate more rapidly the nearer they are to the disk-center, and less rapidly as the periph- 

 ery is approached, thus according with the rule discovered by Morgan in the regenerating 

 fish's fin the deeper the level of the cut the more rapid the regeneration. 



In 1908, Stockard made the interesting discovery that if the medusae be starved while they 

 regenerate lost arms the disk of the medusa shrinks during the process of regeneration, and its 

 rate of decrease is greater the greater the number of removed arms. The regenerating tissue 

 evidently possesses a greater capacity lor absorbing nutriment than does the somatic tissue of 

 the disk itself, and in this respect the regenerating tissue behaves as does that of cancer which 

 grows rapidly even when the normal tissues surrounding it are wasting away. (See Year Book 

 of the Carnegie Institution of Washington, No. 7, p. 131, 1908.) 



