PHYLUM CCELENTERATA 



137 



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, 



eel 



Fig. 101.— Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal, 

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

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

 region extended into the form of a bell; D, vertical, andD', transversa section (along the line a?;) 

 of medusa. The ectoderm is dotted, the endoderm striated, and th^ Anesogloea black, cir. c. 

 circular canal ; ect. ectoderm ; end endoderm ; end. lam. endoderm 'lamella ; ent. cav. enteric 

 cavity ; hi/p. hypostome or manubrium ; mnb. manubrium ; mBr/l. mesoglcea ; mth. moutli ; 

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



i.e. towards the manubrium. The result of this would 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 



