98 MANUAL OF ZOOLOGY SECT. 



calcareous particle or lithite. These are the marginal sense- 

 organs or lithocysts : they were formerly considered to be 

 organs of hearing, and are hence frequently called otocysts : 

 in all probability their function is to guide the medusa by 

 enabling it to judge of the direction in which it is swim- 

 ming. The marginal organs, in this case, may therefore be 

 looked upon as organs of the sense of direction. 



In the description of the fixed Obelia-colony no mention 

 was made of cells set apart for reproduction, like the ova and 

 sperms of a sponge. As a matter of fact, such sexual cells 

 are only found, in their fully developed condition at least, in 

 the medusae. Hanging at equal distances from the sub- 

 umbrella, in immediate relation with the radial canal, are 

 four ovoid bodies (Fig. 43, gon), each containing a mass of 

 cells which are developed either into ova or into sperms. As 

 each medusa bears organs of one sex only (testes or ovaries 

 as the case may be), the individual medusae are dioecious. 



When the gonads are ripe, the sperms of the male 

 medusae are shed into the water and carried by currents to 

 the females, impregnating the ova, which thus become 

 oosperms or unicellular embryos. The oosperm undergoes 

 complete segmentation (Fig. 44, A~F}, and is converted 

 into an ovoidal ciliated body called &planula (G, H). The 

 planula swims freely for a time (ff), and then settles down 

 on a piece of timber, seaweed, etc., fixes itself by one end 

 (K}, and becomes converted into a hydrula or simple polype 

 (Z, M), having a disc of attachment at its proximal end, 

 and at its distal end a manubrium and circlet of tentacles. 

 Soon the hydrula sends out lateral buds, and, by a frequent 

 repetition of this process, becomes converted into the com- 

 plex Obelia-colony with which we started. 



This remarkable life-history furnishes the first example we 

 have yet met with of alternation of generations, or metagenesis. 



