IV 



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 ab)of polype-form; B, polype-form with extended ten- 

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

 region extended into the form of a bell ; D, vertical, and D', transverse section (along the lineal) 

 of medusa. The ectoderm is dotted, the endodenn striated, and the mesoglcea black, cir. r. 

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

 cavity ; hyp. hypostomc or manubrium ; mnb. manubrium ; mst/l. mesoglcea ; villi, mouth ; 

 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 



