Jellyfiskes. 41 



In the common Aurelia aurita^ whose disk can 

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

 there are marginal tentacles and also eight 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 a 

 marginal membrane or veil, such as exists around the 

 mouth of the bell of hydroid medusae (fig. 21, c, D). 



Thread Cells and Nervous System. The stinging 

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

 thread cells, often grouped in batteries on the tentacles. 



When a hydroid is cut in pieces, each piece may 

 produce lost parts and become a perfect animal if a 

 portion of the margin be preserved, but if not, repro- 

 duction 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 medusas are phosphorescent, emitting light 

 from the whole surface, especially from their margins. 



Sub-class 4: Ctenophora, These are more 

 complex pelagic forms, with a degree of bilateral 

 symmetry, and having on the surface eight radial rows 

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

 rowing organs. 



Summary. The Hydrae and jellyfish es which 

 constitute the class Hydrozoa possess a central 

 stomach cavity into which a single aperture of en- 

 trance leads. They are also armed with thread- cells 

 and possess a body wall of two layers. 



