COMPARISON OF MEDUSA AND POLYPE 249 



umbrellar cavity; a mouth will be formed at its extremity. 

 The double walls of the endoderm lamella have fused to- 

 gether over the greater part of the umbrella, but remain 

 separate at its margin and also in four radially disposed lines 

 reaching from the margin to the manubrium. Thus the ring- 

 canal and radial canals are the relics of a space which has else- 

 where been obliterated by the fusion of the endodermic walls 

 containing it. The first four tentacles are formed as out- 

 growths of the margin of the umbrella at the ends of the four 

 radial canals ; the four interradial tentacles are next formed 

 and then the eight adradials. The medusa is now complete, 

 but still attached by a short pedicle to the blastostyle. The 

 cavity in this pedicle is obliterated, the pedicle breaks across 

 and the medusa is set free, escapes through the mouth of the 

 gonotheca, and propels itself through the water by the rhythmical 

 contractions of the umbrella. 



We are now in a position to make a comparison between 

 the structure of a medusa and that of an ordinary hydranth. 

 Imagine a hydranth, like that shown in fig. 52, to have its 

 body compressed in the longitudinal axis and the margins to 

 which the tentacles are attached to be pulled out so as to 

 form a shallow cup. Then it is clear that the body will 

 correspond to the umbrella of a medusa, the hypostome 

 to the manubrium, and the tentacles of the flattened 

 hydranth to the marginal tentacles of a medusa. Further- 

 more, it is clear that the gastrovascular cavity of the com- 

 pressed hydranth corresponds to the gastrovascular cavity of 

 a developing medusa, and that identity will be established 

 by the fusion of the two layers of endoderm, excepting in the 

 regions of the ring-canal and radial-canals, and by a great 

 increase in the thickness of the mesogloea. The hydranth 

 and the medusa, much as they seem to differ from one 

 another, are therefore built upon the same plan, and their 

 several organs are homologous. But the free-swimming 

 medusa differs from the sessile Hydranth in its greater 

 histological differentiation. In the latter there ^are epithelio- 

 muscular ectoderm cells like those of Hydra. In the medusa 

 such cells occur on the sub-umbrella, but at the margins of 

 the umbrella and on the tentacles a more complete division 

 of labour is often established. Part of the' ectoderm cells 

 take up a deeper situation and give rise to a layer of muscle- 



