IV 



PHYLUM CCELENTERATA 



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

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

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



FIG. 89. Diagrams illustrating the derivation of the medusa from the polype. A, longitudinal,. 



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

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

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

 nv. nv\ nerve-rings ; t. tentacle ;. v. 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 



