240 HYDROID POLYPES 



the margin of the bell is continued inwards into a narrow 

 circular shelf, the velum (v). 



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 short hydranth or Hydra-like body with four 

 tentacles (Fig. 55, 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- 

 gloea. 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. c) 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. lam) indicat- 

 ing 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 

 mesogloea : 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, 



