394 The Animal Kingdom 



The polypoid form is either very reduced or completely lacking. Some 

 of these jellyfish attain huge sizes, up to seven or eight feet in diameter, 

 while others are quite small with one inch to four feet being a common 

 range. 



Although superficially the medusoid forms of this class bear a re- 

 semblance to those of the class Hydrozoa, there are several fundamental 

 differences. One of these is the fact that the velum is lacking. Another 

 conspicuous difference is the fact that the mesoglea is much stiffer and 

 contains cells. It gives these jellyfish a somewhat more solid structure 

 than the medusae of the hydrozoans have. Last, the umbrella is divided 

 by septa into four equal areas. The structure and life history of this 

 class are illustrated by the members of the genus Aurellia. 



Aurellia. — This colorful animal with its iridescent bluish white 

 umbrella and pink gonads is commonly seen floating along our coastal 

 areas. At times it is blown into sheltered harbors in large numbers. 

 In size, it is comparatively small, being only about three inches in 

 diameter when fully grown. 



.TENTACLES MANUBRIUM 

 MOUTH, 

 RING CANAL 



ORAL LOBE 

 GONAD 



\i sviijs^ LAPPETS, 



EGG 



STROeiLA 



SCYPHISTOMA 



Fig. 129. — Life history of Aurellia. 



The slender tentacles which are on the margin of the umbrella 

 are interrupted at eight equally spaced points by small indentations. In 

 each of these is a sense organ which is between two small projections 

 or lappets. Each sense organ consists of a pigmented eye spot which is 

 sensitive to light, two pits which are possibly chemical receptors for 



