44 ESSENTIALS OF ZOOLOGY 



of eight of the tentacles. The contrast in this respect between 

 the active sexual generation and the passive nutritive genera- 

 tion is noteworthy. 



The sexes are separate. Both the male and female gonads 

 form prominent oval sacs, and the sex cells lie between the 

 ectoderm and the endoderm. Sex cells cannot be distin- 

 guished until they appear in the medusa. After fertilisation 

 the egg segments and a blastula is formed. This is converted 

 by budding off from the interior into a planula. The planula 

 larva is ciliated, and, escaping from the medusa, swims in a 

 definite direction. The forward end is broad, and after a period 

 of freedom the larva sinks to the bottom and the forward 

 end is, after some exploration, applied to the weed or other 

 object and broadens out to form a base of attachment. The 

 posterior end of the planula, now the free end, develops 

 tentacles and then a mouth, and by growth and branching the 

 stolon and the stems of a new colony are established. Even 

 before it is attached a cuticular secretion of the ectoderm 

 forms, a prelude to the chitinous covering of the animal. 



The medusa of Obelia is very small, measuring only a few 

 millimetres, and in this state a wide dispersion takes place 

 in tidal and aperiodic currents. The pulsations of the bell 

 are sufficient only to control the horizontal position of the 

 medusa, and to carry it across current to some extent. The 

 medusa is altogether unable to swim against the currents of 

 the sea. 



During the spring this small medusa is very common in 

 the shore region, and is procured in plankton nets along with 

 many other beautiful medusae liberated by the species of 

 other genera. 



