IV 



PHYLUM CCELENTERATA 



137 



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

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

 e bent into the form of a saucer (C) with the concavity distal, 



FIG. lOl.-Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal, 

 and A', transverse section (along the line aft) 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 ab) 

 of medusa. The ectoderm is dotted, the endoderm striated, and the mesogloea black, dr. c. 

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

 cavity ; hyp. hypostome or manubrium ; mnb. manubrium ; msgl. mesogkea ; mth. mouth ; 

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



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 



