OBELIA. 33 



Along the radial canals four patches of germ- 

 cells, either male or female, are found. 



Obelia-medusce can be chained by keeping ripe Obelia- 

 hydroids in a dim light for a few days. Frequently also they 

 are taken by using a fine muslin net with a bottle tied to re- 

 place the ordinary bottom of the net, and towed behind a boat. 



They may be fixed with weak (2 per cent.) formalin, or 

 with Flemming's mixture. In hardening, transfer very slowly 

 from one grade of alcohol to another. 



C. Development. 



The fertilised egg of Obelia gives rise to an egg- 

 shaped ciliated body, or ' planula,' composed of an 

 outer and an inner layer of cells. This settles down, 

 and by budding establishes the hydroid. Some of 

 the buds feed ; others have no mouth, and become 

 medusae. The entire product of the development 

 of the egg comprises the hydroid colony and its 

 medusae, and to this the term individual ' is con- 

 veniently given. The individuals so produced from 

 the eggs of medusae of a colony form a new genera- 

 tion. There is, therefore, no ' alternation ' of 

 hydroid and medusoid ' generations ' as is often 

 maintained. The medusa is a specialised bud or 

 organ of reproduction, and may be compared with 

 the flower of a plant if we could imagine such a 

 structure to detach itself and to disseminate its 

 seeds by active or passive movements. 



