NOTES ON BOPYRUS SQUILLARUM. 29 



conception of the relative differences, not only between the typical 

 crustacean as host and its particular parasite under consideration, 

 but also between the different stages of growth of that parasite, 

 I think they will agree with me that no better illustration could 

 be adduced of "the effects of use and disuse of parts," and of 

 the '• adaptation of the organism to its environment." As Mr. Herbert 

 Spencer has pointed out in the " Principles of Biology " before referred 

 to, animals of the Annulose type become unsymmetrical when their 

 parts are unsymmetrically related to the environment. The common 

 hermit crab i Fafiurns > furnishes an illustration like Bopyrus. The 

 embryos of each of these creatures are symmetrical, but the curvature 

 of the body of the hermit crab is due to the position it acquires to 

 adapt itself to the shell which it inhabits, and the unsymmetrical 

 condition of the adult Bopynt^ is similarly due to the position it occu- 

 pies within the carapace of its host the prawn. Except for the 

 writings of Dr. Darwin and Mr. Herbert Spencer * such biological 

 problems as that presented in the morphology and degraded structure 

 of Bopynt.t would be totally inexplicable. Thanks, however, to the 

 light thrown on these questions, especially by the illustrious author of 

 " The Origin of Species," a new- significance is apparent, and as 

 he has shown in that wonderful work, " any change in func- 

 tion which can be effected by insensibly small steps is within the 

 power of natural selection ; so that an organ rendered, during changed 

 habits of life, useless or injurious for one purpose, might be modified 

 and used for another purpose." And again : " Rudimentary organs 

 may be compared with the letters of a word, still retained in the 

 spelling, but become useless in the pronunciation, but which serve as 

 a clue in seeking for its derivation,"! 



Bopyrus, so far as the female is concerned, is apparently getting 

 worsted in the struggle for existence. One cannot help thinking that 

 in the distant future its lease of life will not be remarkably long. So 

 much the better for the prawn ! 



I 



* Vide •• The Principles of Biologj-." By Herbert Spencer, 1867. Vol. 11. 

 page 183, passim. 



t " Origin of Species," pp. 52, 40, Fourth Edition, 18C6. 



