On the Strobilization of Aurelia. 



By 



E. Percival, B.Sc, 



Department of Zoology, The University, Leeds. 

 With Plate 6 and 3 Text-figures. 



1. Introductory and Historical. 



It is well known that in the production of a polydisc strobila 

 the scyphistoma undergoes a series of constrictions as far 

 as the septal longitudinal muscle-strands, commencing just below 

 the wreath of tentacles and continuing downwards towards 

 the foot. There is always a portion at the pedal end which 

 does not undergo constriction, remaining ultimately as a polyp 

 after regeneration of oral disc and tentacles. The animal at 

 this stage is a pile of segments, the oldest and most defined 

 being at the oral end, while the youngest and least defined is 

 near the pedal end. Later, there usually arise from each 

 segment, or ephyra rudiment, eight radiating lobes enclosing 

 diverticula from the enteron, four in the interradii and four in 

 the perradii. At the end of each lobe develops a median out- 

 growth containing an extension of the enteron, which becomes 

 the tentaculocyst. On each side of the tentaculocyst is 

 a wing-like lappet of ectoderm. 



The scyphistoma, just before undergoing the external 

 changes, has four radiating septa perforated each by an ostium 

 beneath the oral disc, forming together a ' gastral ring-sinus '. 

 As the body increases in length new ostia arise below the old 

 ones (Text-fig. 1), and the constriction takes place between the 

 sets of perforations which later take part in the formation of the 

 gastral cavity of the ephyra. 



There appears to be unanimity of opinion between previous 

 workers on this portion of the change, but with regard to later 

 developments there is some diversity of view. Perhaps they 



