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, 



B 



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 ab) of form with tentacular 

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

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

 circular canal; net. ectoderm; end endoderm; on/, lam. endoderm lamella; cnt. cm\ enteric 

 cavity; lujp. hypostome or manubrium ; mnb. manubrium ; m#r2. mesogloea ; mth. mouth; 

 nv. >ii?', nerve-rings ; t. tentacle ; c. 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 



