Jelly fishes and A nemones. 4 1 



In the common Medusa aurita, whose disk can 

 easily be recognised by the four, ring-like, violet ovaries, 

 there are marginal tentacles and also little pigment 

 spots and clear vesicles (eyes and ears) symmetrically 

 disposed around the disk, each covered by a little 

 lobe of the umbrella margin. There is no trace of d 

 marginal membrane or veil, such as exists around the 

 mouth of the bell of hydroid medusae. 



Thread Cells and Nervous System. The stinging 

 power of these * sea nettles ' is due to their armature 

 of thread cells. 



When a hydroid is cut in pieces, each piece be- 

 comes a perfect animal if a portion of the margin is 

 preserved, but if not, reproduction is very uncertain. 



In these medusoids we meet for the first time 

 with an area of sensitive tissue acting like a nervous 

 system, and connected with the margin of the disk. 

 If this be preserved motion continues to take place, 

 whereas if the margin be cut away motion ceases. 



Many medusae are phosphorescent, emitting light 

 from the -^hole surface, especially from the margin of 

 the disk. 



Ctenophora. In one interesting little group,a fourth 

 sub-class, called Ctenophora, there are eight radial rows 

 of comb-like plates armed with cilia, which act as rowing 

 organs. In these there is a certain amount of bilateral 

 symmetry. 



Summary. The Hydrae and jellyfishes which 

 constitute the class Hydrozoa possess a central 

 stomach cavity into which a single aperture of entrance 

 leads. They are also armed with thread-cells and 

 possess a body wall of two layers. 



