CNIDARIA 



119 



the gastral cavity is continued in the form of lobe-pouches 

 (Fig. 60 (J). After the Ephyra shape has thus found ex- 

 pression in the region of the oral pole, the larva shortens 

 in the direction of the chief axis, and gradually assumes the 

 flat, discoid form of the Ephyra. Meantime the larva loses 

 the covering of flagella, and from now on moves like a medusa 

 by the regular contractions of the margin of the disc. In 

 Pelagia accordingly the larva coming from the eo;g passes 

 directly into the Ephyra, although Goette has pointed out 

 that, owing to its structure, we must regard the first stages 

 of this metamorphosis as free-swimming Scyphistoma stages. 



Fig. 60. — Three stages of development of the free-swimming larva of Pelagia 

 noctiluca (after Keohn). r, marginal lobes; s, sensory bodies; m, mouth- 

 opening. 



Metamorphosis of the Ephyra. — The metamorphosis 

 of the Ephyra is accompanied by a constant increase in the 

 size of the body. The sensory bodies of the Ephyra become 

 the eight marginal bodies [rhopalia] of the medusa. Since 

 the adjacent alar lobes, from which the ocellar lohes arise, do 

 not continue to grow with the same rapidity as the rest of 

 the body, new structures, corresponding in position to the 

 adradial regions, are developed in the mai'gin (adradial or 

 intermediate marginal lobes). 



The simplest conditions directly referable to the Ephyra are found 

 in the Ephyropsidffi (Nausithoe), in which the sixteen alar lobes of the 

 Ephyra are retained comparatively well developed, while eight adradial 

 (intermediate) tentacles alternate with these. The pocket-like marginal 

 pouches separated by narrow concrescence-bands (Glaus) and the absence 

 of arm-like prolongations of the angles of the mouth are so many 

 characters derived directly from the Ephyra. In the families of the 



