246 



THE EVOLUTION OF SEX. 



petuated in the various groups a varied series of adaptations 

 securing fecundation. At the same time, the increasing 

 differentiation of the sexes has in the higher animals been 

 enhanced by psychical as well as physical attractions, thus 

 more and more ensuring the continuance of the species. 



Male of Paper Nautilus (Argonauta), with its modified 

 arm.— From Leunis. 



A not unfrequent mode of fecundation is by means of spermatophores, 

 or packets of spermatozoa. These may be seen at times attached to the 

 earth-worm, or found within the leech and snail. Even in newts sperma- 

 tophores are formed, which are taken up by the females. 



In the spider the spermatozoa are stored in a special receptacle on the 

 palp, and hence hastily transferred to the fierce female. In cuttlefishes 



Diplozoon paj-adoxuDi, a douljle organism 

 formed from the union of two distinct 

 hermaphrodite individual trematodes 

 (i9(/tf7;j!>a) at an early stage in their life. 



this mode of impregnation is yet more marked. One of the "arms" of 

 the male, much modified and laden with spermatojihores, is thrust, or in 

 many cases bodily discharged into the branchial cavity of the female, where 

 it bursts. Such a discharged arm was, on first discovery, regarded as a 

 parasite, and hence receivecl the name of Ilectocotylus. A curious aberra- 



