LESS, xxin MEDUSA DERIVED FROM HYDRANTH 243 



A', transverse section of the same through the plane a b. 



B, the tentacular region is extended into a hollow disc. 



C, the tentacular region has been further extended and bent into a 

 bell-like form, the enteric cavity being continued into the bell (ent. cav'} : 

 the hypostome now forms a manubrium (mnb}. 



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

 continuous cavity (ent. cav'} in the bell. 



D, fully formed medusa : the cavity in the bell is reduced to the 

 radiating (rad) and circular (dr. c} canals, the velum (v) is formed, 

 and a double nerve-ring (nv, nv'} is produced from the ectoderm. 



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

 four radiating canals (rad} united by the endoderm-lamella (end. /a), 

 produced by partial obliteration of the continuous cavity ent. cav' in C' 



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

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

 body with bell and manubrium, but with a continuous cavity 

 (c', ent. cav'} in the thickness of the bell 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 bell 

 (D, 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 (end. la) indi- 

 cating the former extension of the cavity. 



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

 manubrium are respectively endoderm and ectoderm : that 

 the gelatinous tissue of the bell is an immensely thickened 

 mesoglcea : that the layer of cells covering both inner and 

 outer surfaces of the bell is ectodermal : and that the layer 

 of cells lining the system of canals, together with the 

 endoderm-lamella, is endodermal. 



Thus the medusa and the hydranth are similarly con- 

 structed or homologous structures, and the hydroid colony, 



R 2 



