154 MEA> T I>'Q OF THE SYSTEM. 



tial aid to locomotion, is reduced more and more till it becomes a 

 short, unsegmented stump. The appearance of such a parasite is so 

 strange that one can easily comprehend how it was that formerly 

 one of these abnormal groups, the Lerncece, was placed among the 

 endoparasitic Worms, or even among the Mollusca. 



The more the female remains behind the type of its fully-developed, 

 free-living allies, so much the more do the two aexes become morpho- 

 logically remote from one another, for the form and organization of 

 the male also are affected by the changed conditions of life, but in 

 a different manner.* In the male sex the more favourable and 

 abundant nourishment may not affect the necessity of locomotion 

 and the development of the locomotive organs in so direct a manner, 

 since the sexual activity of the male and the necessity for locomotion 

 in order to select a female remain unaltered. Even when locomo- 

 tion is reduced and rendered difficult, parasitism, does not, in the case 

 of the male, lead either to a complete loss of segmentation or to such 

 unsymmetrical growths as we observe in many female parasitic Crus- 

 tacea. The large quantity of generative material produced, which 

 in the female is of the greatest importance for the preservation of 

 the species, and which therefore favours the development of a large, 

 shapeless, unwieldy body, .is the less conspicuous in the male because 

 a very small quantity of sperm serves for the fertilization of an 

 enormous number of ova. 



Thus, then, the extreme degree of parasitism in the male, even 

 when accompanied by a confined and more creeping mode of loco- 

 motion, does not lead to an excessive increase in size nor produce 

 an unsegmented and strange form of body, but, on the contrary, 

 gives rise to the symmetrically formed, dwarfed pigmrcan males. 

 This extreme state is, however, connected with the normal state by 

 numerous intermediate steps. Thus we find in the Lernrcopods that 

 the size of the male Adheres is only slightly reduced, while the true 

 dwarfed males of the Lemceopoda and Chondracanthidce are attached, 

 lilje small parasites (fig. 98), to the posterior end of the female body, 

 which is relatively enormously large. The preparation of a large 

 amount of sperm which implies the possession of a large body, would 

 only be a useless expenditure of material and time in the life of the 

 species, and this must have been avoided by the influence of natural 

 selection. 



In addition to this sexual dimorphism we find in various groups 

 of animals especially in the insects which live together in great 



* Compare C. Clans, ' Die freilebcnden Copepoden." 1363. 



