CELETOPODA 293 



arm is exclusively devoted to the copulatory function must 

 profoundly affect its functional activity, and thus in each case 

 there must be great differences in the manner and intensity of 

 the muscular activity of the arm, and in the external irritations 

 to which it is subjected. The suckers are essentially muscular 

 organs, and therefore their disappearance may reasonably be 

 attributed simply to their disuse. A thorough investigation 

 of the relations between the peculiarities of the hectocotylised 

 arm and the mode in which it is used, both in ordinary life 

 and in copulation, would be of great interest, but at present 

 little seems to be known on the subject. 



ClLETOPODA 



The two Orders of animals united in the class Chsetopoda, 

 namely, the Oligochseta and the Polychseta, although re- 

 sembling each other in the essential structure of the chaetae 

 and the regular segmentation or metamerism of the body, 

 differ widely in their reproductive organs and methods. 

 Although it may not be possible in the present state of know- 

 ledge to form a plausible theory in detail of the derivation of 

 the terrestrial and fresh-water Oligochaates from the marine 

 Polychsetes, we may, nevertheless, reasonably regard the 

 generative adaptations of the Oligochaates as necessarily 

 related to their conditions of life. They are all herma- 

 phrodite, and this character does not appear, it is true, to be 

 necessary for the fertilisation or development of the eggs in 

 animals living in fresh water or in. moist earth. But as 

 these hermaphrodites are not self - fertilising some kind of 

 copulation between the reproductive individuals is required 

 to ensure fertilisation, or, to state the matter in more general 

 terms, it is requisite that special means shall be provided to 

 bring the spermatozoa and ova into contact. The means 

 actually employed consist in the emission with the ova of 



