ectodeiin sunk into the supporting substance from the outer sur- 

 face of the proboscis. In these structures nettle cells are 

 to be found in all stages of development. 



The lower disk of the strobila remains simply an annular 

 fold of the body wall until the upper disk is nearly a complete 

 medusa. The septal muscles in this region bend outward with 

 the rest of the body wall. Fig. 49. At length, however, the 

 entoderm grows out as four shallow pouches with gelatinous sep- 

 ta between containing the muscles, anci very soon after the gel- 

 atinous septa are perforated so as to allow a fusion of the 

 entoderm at at their upper armies. Fig. 50. This figure 

 shows that the body wall has become very thin at the bottom of 

 the groove that separates the disks, ana the septal muscles in 

 this region have become constricted. The body wall finally 

 becomes so thin here that it is ruptured by the movements of 

 the upper disk which is then set free, and at about tlie same 

 time the columellae lose their connection with the exumbrella. 



Fig. 19 represents an advanced strobila. The lower disk 

 is drawn as it usually appeared, very much contracted. In oc- 

 casional moments of relaxation a few small tentacles coula be 

 seen but they disappeared again before one could count them. 

 The bua dropped off and began swimming about while I was taking 

 a short rest from drawing. Pulsating contractions of the um- 



49 



