SUBKINGDOM METAZOA. 61 



Schematically such an arrangement may be represented thus, A repre- 

 senting the non-sexual and B the sexual generation : 



/B— A, etc. 

 ,//B— A, etc. 

 ^N\B— A, etc. 



\B— A, etc. 



Among the Hydromedusee, in which group alternation of generations 

 likewise occurs, the process is usually complicated by a number of non- 

 sexual generations succeeding one another before the intervention of the 

 Medusa, thus : 



A'/A" 

 ./'^\A" =;B = A, etc. 

 \i,/A" = B = A, etc. 

 A\A" 



And in some cases the succession is still further complicated by non-sexual 

 reproduction on the part of the medusa, thus : 



A, etc. 

 /A'<(f,', _ l/B' = A, etc. 



\ /A" - b/^' = ^' '*"• 

 ^\A" ~ I \^' = ^' **"• 

 A, etc. 



But such complications do not interfere with the general alternation 

 which invariably occurs in such forms before the completion of the repro- 

 ductive cycle. 



Such a phenomenon as this where a true non-sexual gen- 

 eration alternates with a sexual one presenting a different 

 structure is usually distinguished as metagenesis from another 

 form of alternation of generations known as heterogony, in 

 which the first generation reproduces parthenogenetically, 

 giving rise to a second generation differing in form from the 

 first and reproducing by the sexual method. Typical exam- 

 ples of this process are to be found among the Trematode 

 worms (q-v.), where the sexual worm gives rise to a sporocyst 

 in the interior of which ova, developing parthenogenetically, 

 give rise to a larva which later on transforms to the adult 

 worm. In a less perfect form heterogony occurs in many 

 lower Crustacea (Daphnia), which throughout the warmer 

 portion of the year produce " summer eggs " which develop 

 parthenogenetically, male animals appearing only for a short 

 period in the autumn, as a rule, when the females produce 



