CHAP, v MEDUSA 317 



the enteric cavity being continued into the umbrella (ent. cav'} ; the hypostome 

 now forms a manubrium (innU). 



C', transverse section of the same through the plane ab, showing the continuous 

 cavity (ent. cav' ) in the umbrella. 



D, fully formed medusa ; the cavity in the umbrella is reduced to the radial 

 (rod) and circular (dr. c) canals, the velum [(v) is formed, and a double nerve- 

 ring (>, nv') is produced from the ectoderm. 



D', transverse section of the same through the plane <5, showing the four radial 

 canals (racf) united by the endoderm-lamella (end. lani\ produced by partial 

 obliteration of the continuous cavity (ent. cav') in C. (From Parker's Biology.) 



there is no extension of endoderm into it. The tentacles, 

 like those of the polype, are formed of a core of endoderm 

 covered by ectoderm, which encloses numerous stinging- 

 capsules. 



At first sight there appears to be very little resemblance 

 between a medusa and a hydranth, but it is really quite 

 easy to derive the one form from the other. 



Suppose a simple polype or Hydra-like body with four 

 tentacles (Fig. 80, A, A') to have the region from which the 

 tentacles spring pulled out so as to form a hollow, trans- 

 versely extended disc (B). Next, suppose this disc to become 

 bent into the form of a cup with its concavity towards the 

 hypostome, and to undergo a great thickening of its meso- 

 glcea. A form would be produced like c, t.e., a medusa-like 

 body with umbrella and manubrium, but with a continuous 

 cavity (c', ent. cav'] in the thickness of the umbrella instead 

 of four radial canals. Finally, suppose the inner and outer 

 walls of this cavity to grow towards one another and meet, 

 thus obliterating the cavity, except along four narrow radial 

 areas (D, rad) and a circular area near the edge of the 

 umbrella (dr. c). This would result in the substitution 

 for the continuous cavity of four radial canals opening on 

 the one hand into a circular canal and on the other into 

 the cavity of the manubrium (ent. cav\ and connected with 

 one another by a membrane the endoderm-lamella (D', end. 

 lam) indicating the former extension of the cavity. 



It follows from this that the inner and outer layers of the 



