Unicellular and Multicellular Animals 



in the manner already described, forming a new colony. The 

 process that we have described is that of sexual reproduction, and 

 its essential features are the same as in Volvox throughout the 

 whole animal kingdom. The small free-swimming cells are the 

 male reproductive bodies or sperms, the others are the female or 

 egg cells. The union of the two produces the fertilised egg, and 

 the process of union is termed fertilisation. In Volvox, the male 



- 



FIG. 30. Volvox. Portion of a hermaphrodite individual, 

 showing egg cells (0, Oi\ and sperms (Si S 2 S 3 ). 



and female elements are sometimes produced by the same in- 

 dividual, at other times by different ones. Separation of the 

 sexes is no necessary accompaniment of the process of sexual re- 

 production, and indeed it is only in the higher groups of animals 

 that separate sexes are the rule. The various conditions in 

 Volvox are illustrated in Figs. 28, 29, and 30. 



The next great groups of animals are, on the one hand, that 

 of the sponges, and, on the other, that which includes the sea- 



39 



