100 PLAIN FACTS FOR OLD AND YOUNG 



effort to puncture its assailant in a similar manner. 

 The darts are generally broken off in this encounter, 

 and either fall to the ground, or remain fixed in the 

 wounds which they have inflicted. After these pre- 

 paratory stimulations, the snails proceed to more ef- 

 fective advances. The sac of the dart is withdrawn 

 into the body, and another sacculus is by a like proc- 

 ess protruded from the common generative aperture. 

 Upon the last-named organ, two orifices are seen, one 

 of which leads to the female generative system, while 

 from the other a long and whip-like penis is slowly 

 unfolded, being gradually everted like the finger of a 

 glove, until it attains the length of an inch or more. 

 Then each of the two snails impregnates its partner, 

 and is itself impregnated at the same time." 



In the oyster, another hermaphrodite, self-fecun- 

 dation occurs. In the argonaut, a species of cuttle-fish, 

 fecundation is effected in a most extraordinary manner. 

 The male, which is smaller than the female, has upon 

 the left side of its body a little sac, in which grows a 

 coiled-up, worm-like arm covered with suckers. This 

 arm is really a sac, which communicates with the tes- 

 tes, and contains spermatozoa. On reaching full devel- 

 opment, and becoming filled with spermatozoa, this 

 curious arm detaches itself from the body of the argo- 

 naut, and begins an independent life. Floating through 

 the water, it by and by finds a female argonaut, with 

 which it connects itself, and impregnates it with the 

 spermatozoa transported from the male. 



In the tapeworm, a parasitic creature which is found 

 in the human digestive canal, a very curious form 

 of fecundation has been noted. A\^ien liberated from 

 the egg, it consists simply of a head with hooks, by 

 which it attaches itself to the mucous membrane of the 



