HYDROIDS AND JELLY-FISH 47 



The Scyphozoa include the larger and commoner kinds of 

 Jelly-fish, hundreds of which may be seen during the summer 

 months swimming along by gracefully expanding and contracting 

 their umbrella-shaped disks, or drifting quietly along the tidal 

 currents. At night many of them present a very beautiful appear- 

 ance, becoming lit up by a soft yet brilliant phosphorescence, 

 veritable living lamps, glowing with many-coloured lights as they 

 rise from out the dark, mysterious depths. 



One of the commonest of these large Jelly-fish, often cast up 

 in large numbers on the shore, is the Aurelia, 1 easily recognised 

 by its saucer-shaped umbrella measuring 3 or 4 inches in 

 diameter with four red or purple horseshoe-shaped bodies (the 

 reproductive masses) embedded in the jelly near the centre of the 

 body. The margin of the umbrella is fringed by very short, fine 

 tentacles, and indented at regular intervals by a series of eight 

 notches, each containing a little cyst, which is regarded as a pri- 

 mitive eye-spot and is covered by a pair of minute flaps. Hence 

 these larger Jelly-fish have been called " Covered-eyed Medusae," 

 to distinguish them from the Hydroid type, which are without these 

 flaps, and were called " Naked-eyed Medusae." The mouth is in 

 the centre of the under surface of the body, and from it project four 

 groups of gastral filaments well armed with thread or stinging 

 cells. The fertilised egg of Aurelia develops into a hollow oval 

 embryo, covered with cilia by means of which it moves through 

 the water. It soon settles down, however, and becomes attached 

 at one end to some base on the floor of the sea, where it changes 

 into a small polyp with a mouth, stomach, and sixteen tentacles ; 

 it was formerly thought at this stage to be a distinct individual, 

 and received as such the name of Hydra tuba, or the " Trumpet 

 Polyp." After feeding for a while, the little polyp begins to show 

 a series of transverse constrictions or waists, which become smaller 

 and smaller until the little creature looks like a pile of saucers 

 with deeply indented edges. In due course the little saucers 

 detach themselves and swim away, their transformation being 

 completed by the filling in of the spaces between the eight deep 

 indentations of the margin of the saucer, and the development 

 of the fringe of fine tentacles, when the little creatures are 

 recognised as small Aurelias. 



1 Aurelia aurita* 



