ORIGIN OF THE INDIVIDUAL 



219 



The medusae, which become detached and swim away, 

 usually bear no superficial resemblance to any of the other 

 individuals of the colony on which they arose, but a study 

 of their structure shows that they are built on the same 

 fundamental plan and are, to all intents and purposes, free- 

 swimming sexual hydranths, some of which produce sperm 

 and others eggs. The medusae liberate their sexual products 

 in the water where fertilization occurs, and the zygote gives 



FIG. 119. Diagrams to show the fundamentally similar structure of Hydra or 

 of a hydranth of Obelia (A) and of a medusa (B). circ, circular canal; ect, ectoderm; 

 end, endoderm; ent. cav, enteric cavity; hyp, mnb, region of mouth (mth); msgl, meso- 

 gloea; nv, nv l , nerve rings; rad, radial canal; v, velum. (From Parker.) 



rise to a free swimming embryo (LARVA) . This soon becomes 

 attached to some submerged object and develops into a Hy- 

 droid colony. (Figs. 118, 119.) 



Thus the common Hydroids, such as OBELIA, exhibit two 

 distinct phases, or generations, in their life history the 

 fixed, polymorphic colony of hydranths, or polypes, which is 

 produced sexually but is itself asexual; and the free-swim- 

 ming medusae which are produced asexually but are them- 

 selves sexual. The asexual and sexual generations alternate 

 with each other in regular sequence, so that an alternation of 



