24 



BRANCH CCELENTERATA 



The nerve ring surrounds the margin between the circular 

 muscles of the umbrella and those of the velum. At the bases 

 of two of the tentacles of each quadrant there are sense organs. 

 They probably aid the medusa in determining in what direction, 

 with regard to the vertical, it is swimming, that is, whether it is 

 moving up, down, or sidewise. In other medusae the simplest 

 of eyes, red pigment spots, which may or may not have a lens, 

 are found. 



The food of the medusa consists of both plants and animals. 

 It is very voracious and grows rapidly after leaving the colony. 



Nutritive individual; b, reproduc- 

 (Bull. 455, U. S. 



c. a- 



Fig. 15. — Hydractin'ia polydi'na 

 tive individual; c, spiral zooids or fighting individuals. 



F. C.) 



Multiplication.— After a time either eggs or sperm cells 

 develop, and are set free in the water, where they unite with 

 those of some other medusa and develop into the tiny larval 

 form, which soon attaches itself and grows into a hydroid, to 

 bud and ])ranch and produce again the medusse, thus repeating 

 the life-cycle and the reproduction by alternation of genera- 

 tions. 



There are more than a thousand species of the class Hydrozoa. In 

 some forms (Fig. 15), as the Hy'dractin'ia, there are several classes of 

 individuals— the nutritive, the defensive, and the reproductive— with 



