NORMAL MOVEMENTS IN SEA-WATER. 7 



II. PULSATION OF CASSIOPEA IN SEA-WATER. 



INTRODUCTION — NORMAL MOVEMENTS. 



The rhizostomous Scyphomedusa Cassiopea xamachana (plates i, 

 ii), is very abundant during spring and summer in the salt-water moat 

 of Fort Jefferson, at Tortugas, Florida. It was described by Bigelow 

 (1892, 1900) from a salt-water lagoon in Jamaica, and also under the 

 name of Cassiopea frondosa by Fewkes (1883), who found it at the 

 Tortugas. 



The Medusae are usually found gathered in clusters upon the weedy 

 bottom of the moat in water about four feet deep. They lie with the 

 aboral side of the disk pressed downward upon the bottom, and with 

 the 8 mouth -arms, with their numerous suctorial mouths, spread out 

 above. A sucker-like concavity on the aboral side of the disk allows 

 the Medusa to adhere with considerable strength to the bottom or sides 

 of an aquarium, and the tenacity of its hold is still further enhanced 

 by the rhythmical movement of the disk, which beats with considerable 

 regularity, thus tending to hold the bell firmly against its fastening, 

 and also to drive a current of water out over the mouth-arms. 



If moved from its normal position and placed in the water with its 

 disk uppermost and arms downward, the rhythmical beating of the 

 disk causes it to swim upward, but if the water be of considerable 

 depth it soon topples over and thus swims downward to the bottom 

 or reaches the side of the aquarium. If, however, it should reach the 

 surface, the concavity at the center of the aboral side of the disk often 

 serves to permit the surface tension to hold the Medusa upon the sur- 

 face, where it may float for a long time, pulsating normally with the 

 concavity relatively dry, although lower than the general surface of 

 the water. 



The Medusa pulsates with a regular rhythmical movement, pauses or 

 irregularities in the rhythm being exceptional. Occasionally, how- 

 ever, its rate suddenly increases, with or without apparent cause, and 

 the pulsation may become so active as to cause the Medusa to break 

 away from its anchorage and glide over the bottom. A regular unex- 

 cited movement is, however, often maintained for hours at a time, and 

 in general this rate of pulsation is faster in small than in large Medu- 

 sae, as will appear from table i, on page 8. 



The relatively rapid rate of small Medusae is probably due to their 

 being young and possessed of more vitality than are the large, old 

 animals ; for not only do small Medusae regenerate lost parts more 

 readily, but we also find that specimens which have become reduced 

 in size through starvation pulsate at a slower rate than young and 



