IV 



PHYLUM CCELENTERATA 



127 



the two cases. Suppose the tentacular region of a polype to be 

 pulled out, as it were, into a disc-like form (B), and afterwards to 

 be bent into the form of a saucer (C) with the concavity distal. 



ecf 



B 



eel 



i IQ. 89. Diagrams illustrating the derivation of the medusa from the polype. A, lungitudinal, 

 and A', transverse section (along the line 06) of polype-form ; B, polype-form with extended ten- 

 tacular region ; C, vertical, and C', transverse section (along the line ab) of form with tentacular 

 region extended into the form-of a bell ; D, vertical, and D', transverse section (along the line cd>) 

 of medusa. The ectoderm is dotted, the endoderm striated, and the mesoglcea black ; dr. <. 

 circular canal; ect. ectoderm; end endoderm; end. lain, endoderm lamella; >,ni. cav. rut 

 cavity; hyp. hypostome or manubrium ; ninb. manubrium ; instil, mesoglcea ; mlfi. mouth; 

 nv. nv', nerve-rings ; t. tentacle ; i: velum. (From Parker's Biology.) 



i.e. towards the manubrium. The result of this will be a medusa- 

 like body (C, C') with a double wall to the entire bell, the narrow 

 space between the two layers containing a prolongation of the 



