ALTERNATION OF GENERATIONS 121 



" jelly-fish " are more definitely bell-shaped, but the manu- 

 brium lies in the hollow of the bell, like its clapper; in comparison 

 with these the Obelia medusa may be said to turn itself inside out 

 in the act of swimming. In accordance with their active life the 

 medusae have a fairly well developed nervous system and a number 

 of sense organs around the margin of the bell. The point with 

 which we are immediately concerned, however, is their mode of 

 reproduction. 



Unlike the hydranths and blastostyles of which the Obelia 

 colony is composed the medusae are sexual individuals, bearing 

 sexual organs or gonads from which the gametes are set free. 

 The gonads (Fig. 60, B, gon.) are masses of germ cells lying one 

 beneath each radial canal, on the same side of the disk as the 

 manubrium. The sexes are distinct, the medusae being either 

 male or female, and the gonads accordingly producing either 

 spermatozoa or ova, which, when mature, are discharged into the 

 water by rupture of the gonad. 



The ova are fertilized by the spermatozoa and undergo 

 segmentation much as in the case of Hydra, developing 

 ultimately into a hydroid individual (hydranth), which by 

 budding gives rise to a new Obelia colony. 



It is obvious that we have here a case of alternation of sexual 

 and asexual generations (metagenesis), comparable to that which 

 occurs in the fern, except that the sexual generation arises from a 

 multicellular bud and not from a unicellular spore. The sexual 

 medusoid individual is produced by budding from the asexual 

 hydroid, and itself gives rise, through the conjugation of gametes 

 and the development of the zygote thus produced, to the asexual 

 hydroid again. The process is somewhat complicated by the fact 

 that the hydroid also produces other asexual individuals by bud- 

 ding, but this really amounts to little more than does the forma- 

 tion of numerous branches by the budding of one of the higher 

 plants, for in the latter case also each bud may be regarded as, 

 potentially at any rate, a perfect individual. 



We may draw an even closer comparison between the alterna- 

 tion of hydroid and medusoid generations in the Ccelenterata and 

 that of sporophyte and gametophyte in the higher plants, for in the 

 former, as in the latter, we see in many instances a more or less 

 strongly developed tendency towards the suppression of the sexual 

 generation (sometimes called the gamobium in animals). 



In Tubularia, for example,' the medusoid individual never 



