402 PHYSIOLOGY OF THE INVERTEBRATA. 



organism ; and it is illustrated by the development of various 

 forms of Medusa^ Twnia, and of Aphides. 



Sexual reproduction. — This mode of reproduction, or 

 gamogenesis, is the result of the fusion of two distinct 

 elements — a male element, or spermatozoon, and a female 

 element or ovum. These are differentiated cells, produced 

 in special organs, of the parent or parents, and by their 

 coalescence a series of changes take place, which ultimately 

 give rise to a new organism. These elements ( $ and $ ) may 

 be produced in the same individual (as in many Annelida and 

 Mulluscd) : such a condition is termed hermaphroditism ; but 

 in the majority of the Invertchrata the male and female organs 

 are on different individuals, in other words, the sexes are 

 completely separate. 



Prof. Huxley states that it is probable that hermaphro- 

 ditism " was the primitive condition of the sexual apparatus, 

 and that unisexuality is the result of the abortion of the 

 organs of the sex, in males and females respectively." 



Although some Invertebrates have both sexual organs on 

 the same individual, these organs are often so arranged that 

 self-fertilisation is almost impossible. As already stated 

 certain Mollusca and Annelida are hermaphrodites, but these 

 all pair, Darwin* states that he had "not found a single 

 terrestrial animal which can fertilise itself. This remarkable 

 fact, which offers so strong a contrast to terrestrial plants, is 

 intelligible on the view of an occasional cross being indispens- 

 able ; for owing to the nature of the fertilising element there 

 are no means, analogous to the action of insects and of the 

 wind with plants, by which an occasional cross could be 

 effected with terrestrial animals without the concurrence of 

 two individuals. Of aquatic animals, there are many self- 

 fertilising hermaphrodites ; but here the currents of water 

 offer an obvious means for an occasional cross." Darwin also 

 remarks that he failed " to discover a single hermaphrodite 

 animal with the organs of reproduction so perfectly enclosed 

 * Origin of Species (6th ed.), p. 79. 



