CHAP, v MEDUSAE 317 



the enteric cavity being continued into the umbrella (enl. caxf) ; the hypostome 

 now forms a manubrium (mnb). 



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

 cavity (ent. can) 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 (tn. tut*) is produced from the ectoderm. 



iy, transverse section of the same through the plane a b, showing the four radial 

 canals (rod) united by the endoderm- lamella (end. lam), produced by partial 

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



middle layer of mesogloea with ectoderm on either side : 

 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 re- 

 semblance between a medusa and a polype, 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, 

 transversely 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 thicken- 

 ing of its mesogloea. A form would be produced like 

 C, i.e., a medusa-like body with umbrella and manu- 

 brium, 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 



