TECHNIQUE OF THE EXPERIMENTS AND SOME RESULTS OF TIONS THAT APPLY EQUALLY TO ALL EXPERIMENTS.
The natural habitat of Cassiopea is in shallow lagoons, usually in areas where mangroves are abundant, where the water is daily subjected to marked fluctuations in temperature, and where the salinity is most affected by precipitation. In such locations, many of which are practically stagnant, the variations in salinity are of wide range, as evaporation is rapid during the summer months, when also the precipi tation is greatest. Such lagoons usually support an abundant algal flora, so that the gaseous content of the water varies considerably and large quantities of organic acids are continually being generated. The environmental conditions consequently vary widely from time to time and are always decidedly different from those in the open tropical ocean. Indeed, so thoroughly inured to this environment is Cassiopea that it thrives better under laboratory or lagoon conditions than in pure sea-water.
At Tortugas the proper environment for this form is restricted to the moat, at Fort Jefferson, which is 50 feet wide, extends entirely around the fort, and communicates with the open water outside at only two points through narrow entrances which are entirely cut off at extreme low tide. As the mean rise and fall of tides is in this region only about 1.5 feet, the amount of change in the water of the moat is slight and currents are scarcely detectable at a distance of 200 yards from the entrances. Over the greater portion of the moat, the bottom is densely covered with a mat of filamentous altos, while its side walls furnish a place of attachment for innumerable specimens of hydroids, bryozoa, tunicates, annelids, mollusks, and several species of corals. Among the alga the cassiopeas lie on their aboral surfaces, with their branching mouth-arms giving a flower-like appearance and with the bell-margin pulsating slowly. In adult specimens, movement from place to place by their own activity apparently occurs very rarely, if at all. Even young medusa, not more than 2 cm. in diameter,
seldom are raised from the bottom by their own pulsation, and it is doubtful if the larger specimens are capable of moving about by swimming movements of the disk.
The exumbrella surface is depressed in the center and can be used as a sucking disk by which a medusa can attach itself firmly to a vertical surface or resist removal from the bottom. This surface has a simple layer of epithelial cells over the mesoglcea and is not provided with either nerves or muscle-cells.
When resting in its normal position on the bottom, the languid pulsations of the bell-margin create currents sufficient to bring the food material (which consists entirely of minute organisms) onto the mouth arms, where it is taken in through the numerous small openings into which the original oral cavity has been subdivided by the branching and anastomosing of the oral arms.
When brought into the laboratory the meduste may be kept in aquaria through which a small stream of water is run slowly—or, if only 3 or 4 specimens are placed in a 10-liter jar, they will remain in normal condition if the water is changed once every 48 hours. The unusual agitation of the water attendant upon changing that in the jars or the handling of the specimens causes a copious secretion of mucus from glands situated on the oral arms, so that the less often they are disturbed the more nearly normal they will remain.
Specimens kept under either of the conditions just mentioned gave the expected results when used for any of the experiments, but in order that any possible source of error from differences of previous treatment might be eliminated, no specimens were used for routine experiments which had been in aquaria more than 12 hours before the time of the first operation.
In all experiments which were to extend over more than a few hours, each disk (or its two separated halves) was kept in a jar containing not less than 4 liters of sea-water.