FORMATION OF THE MEDUS.E. 127 



siderable powers of perception, and some strength and agility, 

 are formed but of a few delicate tissues filled with a fluid to 

 all appearance not very different from sea water, and shrink- 

 ing to a mere nothing when deprived of their vital power. 

 Thus of a Medusa or Rhizostoma weighing from twenty to 

 thirty pounds but few traces remain after death, the ground 

 is covered with a light varnish, all the rest has been absorbed 

 by the thirsty sands. 



The motion of the Discophora is performed by alternate con- 

 tractions and expansions of the umbrella, repeated at regular 

 intervals, something like the movement of the lungs in respira- 

 tion, each contraction forcibly expelling a jet of water, which, 

 impinging on the surrounding fluid, causes a reaction, through 

 which the animal is forced forward with a jerk in the opposite 

 direction. By contracting the whole or only part of its disk, 

 the Medusa has it in its power to direct its movements, and 

 while thus swimming along with the convex end of the um- 

 brella directed forwards, and its fimbriated vessels and tentacula 

 streaming behind, it may well rank among the most elegant 

 children of the sea. Nor are its long thread-like tentacles mere 

 accessorial ornaments or innocent organs of touch, for they are 

 generally armed with the same ' urticating organs ' which render 

 the Actinias and many other polyps formidable to the smaller 

 sea- animals, and after having thus paralysed resistance, are 

 admirably adapted for curling round their prey, and conveying 

 it by their retraction to the mouth. 



But the most remarkable passage in the history of the Disco- 

 phora is the wonderful alternation of their generations. The 

 Medusa first gives birth to a multitude of minute gelatinous 

 bodies, in shape not unlike the eggs of a sponge, or the spores 

 of one of the lower algse, and like them furnished with a mul- 

 titude of cilia, which by their motion propel the little body 

 through the water. When emitted, the bud is of an oval shape, 

 broader at one end ; and it constantly keeps its broader end in 

 advance when moving. Internally it presents a cavity, so that 

 it is in fact a little bag of living jelly clothed with vibratile 

 hairs. After a while the bud attaches itself by its larger ex- 

 tremity or apparent front to any convenient object, such as a 

 stone or the stalk of a sea-weed, and this extremity henceforth 

 becomes the base on which all its future operations are con- 



