126 Papers from the Department of Marine Biology. 



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 medusae 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. 



As most of the experiments required the handling of the specimens 

 under observation once each day they were transferred to clean jars 

 of fresh sea-water at that time. 



The nervous stimulus causing pulsation originates in the sense- 

 organs, probably through a chemical reaction liberating sodium, at the 

 nerve-centers (Mayer, 1908). At any given time only a single sense- 

 organ, which at that moment is discharging at the highest rate, controls 

 the rate of pulsation. Consequently all the nerve-centers except one 

 may be removed without seriously interfering with the activities of the 

 medusa; when the last one has been removed the specimen will remain 

 quiescent until a new sense-organ has been regenerated with sufficient 

 completeness to take up again the normal chemical reactions which 

 liberate sodium. 



Any portion of the disk adjacent to where the nerve-centers have been 

 removed may be insulated from the influence of the remaining centers 

 by destroying the continuity of the subumbrella ectoderm, in which 

 alone the muscles and nervous elements are contained, while still 

 retaining its continuity with the remainder of the disk through the 

 mesogloea. This mesoglcea is of sufficient thickness to afford a support 

 for isolated areas of the active tissues even when almost the entire 

 ectodennal covering has been removed. Also when the medusae are 

 kept in normal sea- water it is little subject to bacterial action and can 

 be maintained in an apparently healthy condition even when consider- 

 ably more than half of the ectodermal layer has been removed. 



