THE SEA-ANEMONE AND SOME ALLIES 269 



sense-organs, which have the function of controlling, through 

 the nervous and muscular systems, the direction of movement 

 of the jellyfish. The most important muscle is a circular 

 band near the rim. The nerve-cells are grouped near the 

 sense-organs, and connect the latter with the muscle-band. 

 When the circular muscle contracts, the bell becomes more 

 convex, and the resulting action of the water in the hollow 

 of the bell against the water outside, sends the animal along 

 in a slow, periodic, pulsating movement. 



The development of Aurelia is very different from that of 

 the hydro ids. The simple, free-swimming larva of Aurelia 

 sinks to the bottom, attaches itself to some fixed object, 

 and takes on a form resembling Hydra. The formation of 

 circular grooves below the tentacles develops a series of saucer- 

 like divisions, one within another. These separate and, swim- 

 ming away with the convex, aboral surface uppermost, grow 

 into the adult form. The fixed stage is comparable with the 

 hydroid generation of Hydrozoa. 



Aurelia is but slightly more dense than the sea-water itself. 

 According to one authority there is less than one eighth of one 

 per cent of proteid material present. Thus over ninety-nine 

 per cent is water. The small amount of proteid material in 

 the body of Aurelia makes it very unlikely that many animals 

 depend on them for food. After a storm in summer great 

 numbers of Aurelise may be found on the shore. In a few 

 hours only scattered films are left on the sand to show where 

 the animals lay. The destruction of many Aureliao is to be 

 expected because of their general helplessness, but annihila- 

 tion of the species is prevented by the enormous number of 

 young produced. 



The great fecundity of Aurelia and its giant relative Cya'nea 

 which sometimes grows to a diameter of eight feet, might 

 threaten to fill the ocean with their bodies, were it not for 

 the fact that neither lives over the winter. The young start 



