138 



ZOOLOGY 



SECT. 



While both polype and medusa are radially symmetrical, the increase in 

 complexity of the medusa is accompanied by a differentiation of the structures 

 lying along certain radii. If a polype is projected on a plane surface (Fig. 

 103, A), taken at right angles to its long axis, a large number of radii about 



fcf 



B 



FIG. 102. Diagram illustrating the derivation of the medusa from the polype. A, longi- 

 tudinal, and A', transverse section (along the, line ab) of polype-form ; B, polpye-form with 

 extended tentacular 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', trans- 

 verse section (along the line ab) of medusa. The ectoderm is dotted, the endoderm striated, 

 and the mesoglcea black, dr. c. circular canal ; ect. ectoderm ; end. endoderm . ; end. lain. 

 endoderm lamella ; ent. cav. enteric cavity ; fiyp. hypostome or manubrium ; mnl>. 

 manubrium ; msgl. mesoglcra ; mth. mouth ; nv.,nv', nerve-rings ; t. tentacle ; v. velum. 

 (From Parker's Biology.) 



twenty -four can be drawn from the centre outwards, all passing through 

 similar parts, i.e. along the axis of a tentacle and through similar portions of 

 the body and manubrium. But in the medusa (B) the case is different. The 

 presence of the four radial canals allows us to distinguish four 2M'i'<'ip<il radii 



