STROBILIZATION OF AURELIA 91 



endoderm. It is, for a time, a flattened plate having a central 

 opening into the enteron. This change does not involve the 

 appearance of a ' proboscis ring-sinus ' as described and figured 

 by Claus, but it is possible that the solid rod of endoderm cells 

 and the ring-sinus may be developed under different conditions. 



Peristomial Pit. — Associated with the development of 

 the manubrium is the formation of the peristomial pits. As 

 has been stated, the constriction in the interradius brings the 

 ectoderm very close to the longitudinal strand (they are 

 separated by a thin layer of mesogloea), and when the endo- 

 dermal plug pushes out the ectoderm a small interradial funnel 

 is formed in the angle made by the fold and subumbrella 

 (PI. 6, fig. 9, P.P.) ; this is the rudiment of the peristomial 

 pit. The subsequent curling outwards and growth of the 

 manubrium and the increase in depth of the ephyra l)ring about 

 the deepening of the funnel (PI. 6, tig. 10, P.P.). 



Longitudinal Muscle. — Contrary to wdiat Claus, 

 Friedemann, and Heric have maintained, transverse sections 

 of a scyphistoma show that the septal muscles may be hollow 

 structures (PL 6, fig. 8). The cavity may not be continuous 

 along the whole length, but it generally appears a short distance 

 below the peristomial pit, and, as Friedemann has shown, 

 the end of the muscle and the apex of the pit are not con- 

 tinuous with each other. The muscle-cells are arranged with 

 their tails to the mesogloea and the protoplasmic parts towards 

 and projecting into the lumen (PI. 6, fig. 8). There are mesogloeal 

 ridges projecting into the cavity and on these are set muscle- 

 cells, and this probably serves to increase the amount of 

 muscular surface. Sometimes the cavity is quite w^ide, and 

 none would describe such a structure as a solid muscle. Towards 

 the foot the muscle-cells are usually closely packed, and this 

 portion of the strand can be described as solid. 



In a strobila, mainly in the lower segments, sections show 

 that some parts of the muscles may be hollow, though the 

 cavities in such cases are short, extending along the depth 

 of an ephyra rudiment. As an ephyra becomes more developed 

 and the circular and radial muscles begin to function, the 



