128 TWO-LAYERED ANIMALS— PHYLUM COELENTERATA 



helpful relationship, each contributing to and each benefiting from the 

 association. Other examples will be mentioned as we go on in our study. 



Some of the smaller jellyfish are also in the order Hydrozoa. A 

 jellyfish gets its name from the fact that the mesoglea is highly developed 

 and, if the epidermis is broken, this jelly-like material will ooze out. 

 Gonionetnus is a good example of one of the smaller ones. It is somewhat 

 umbrella-shaped with long tentacles, bearing the nematocysts, hanging 

 down from the outer rim while the mouth is at the lower end of the 

 handle of the umbrella so that it might be compared roughly to a hydra 

 that has been turned upside down and given a hard slap on its base so 

 as to spread it out to form the umbrella shape. This spread-out portion 

 which curves downward at its outer edge is called the bell. The animal 

 swims by rhythmic pulsations of the bell. The water is slowly sucked up 

 into its undersurface, like air swelling out a parachute, and then is sud- 

 denly expelled as the bell contracts, thus propelling the animal in the 

 opposite direction. This is the original jet propulsion which the jellyfish 

 have been using for thousands of years before man thought of its use in 

 connection with aircraft. A jellyfish swimming through the water ex- 

 hibits one of the most beautiful illustrations of poetry of movement as 

 the tentacles wave back and forth in unison with the pulsations of the 

 bell. 



Like obelia, gonionemus has alternation of generations, but the polyp 

 stage is quite small in comparison with the medusa stage. The polyp of 

 gonionemus looks very much like hydra and the medusa bud off at the 

 side in this asexual phase of reproduction. 



There are other small jellyfish that live in fresh water, and some inland 

 lakes may be filled with them at certain seasons of the year. At other 

 seasons they seem to have disappeared entirely, but a careful search will 

 reveal them in the less conspicuous polyp stage. 



Class — Scyphozoa 



Members of the class Scyphozoa are large marine jellyfish which may 

 reach a diameter as large as two feet. They closely resemble the hydro- 

 zoan jellyfish in its medusa stage and have a very inconspicuous or no 

 polyp stage whatsoever. The gonads and other internal organs can be 

 seen forming a beautiful geometrical pattern through the transparent 

 outer covering and as they float on the surface of the water they look 

 more like a beautiful piece of lace than anything that could do a person 

 any bodily harm. Many a newcomer to the seashore has picked up these 

 beautiful creatures, while in bathing, to examine them more closely, only 

 to drop them quickly as the powerful nematocysts come in contact with 



