108 HALF AN HOUR WITH JELLY-FISH. 



The change which ensues transforms these flat-look- 

 ing objects into a pear-shape. It now settles down 

 to a sedentary existence, attaching itself to some 

 submarine rock, and hanging downwards. A de- 

 pression gradually forms, and tentacles begin to be 

 protruded. In this stage it is the well-known Hydra 

 tuba an animal common in marine aquaria. Its 

 height, however, is only about one-sixth of an inch. 

 What is most singular is that it not unfrequently 

 lives even for years in this condition, without showing 

 signs of a change. Eventually we find the body 

 enlarging in thickness and length, and we see 

 segments beginning to form. Once more it arrives 

 at a stage apparenty generically distinct from any- 

 thing else, and here it was known to naturalists as 

 Scyphistoma. The segments deepen, and look like a 

 pile of saucers, each of which has serrated edges. 

 Again it has been mistaken for another and distinct 

 animal, and named Strdbila. Finally, as the con- 

 striction has gone on, the segments break away, 

 turn over, and become recognisable as true medusae, 

 having a diameter, however, of only the sixth of an 

 inch, but capable of enormous growth subsequently. 

 Their ova settle down into the small and insignificant 

 Hydra tuba. Not unfrequently the phosphorescence 

 of the sea is largely due to the immense numbers of 

 these little medusae. 



Along the eastern and north-eastern coasts of 

 England, there may often be met with a large jelly- 

 fish known as Cyanea chrysaora (Fig. 56). This also 



