282 ILLINOIS ACADEMY OF SCIENCE 



explanation for the development of dimorphism among 

 the lower sexual animals. In the Descent of Man (Chap- 

 ter 9) he states that among the lower organisms "it is 

 almost certain that these animals have too imperfect 

 senses and much too low mental powers to appreciate 

 each other's beauty or their attractions, or to feel ri- 

 valry." It becomes a matter of considerable interest, 

 then, to examine some of the lower bisexual organisms 

 in which mating does occur in order to see if it is possible 

 to offer any explanation of the factors causing dimor- 

 phism under such conditions. 



In the course of work upon the Acanthocephala, I have 

 come across a number of instances of sexual dimorphism 

 which, because of the conditions under which they occur, 

 seem to offer some interesting obstacles to the operation 

 of sexual selection or of natural selection in their de- 

 velopment. Before discussing the specific instances, a 

 few facts regarding these organisms should be enumer- 

 ated. The Acanthocephala are worms of uncertain phylo- 

 genetic relationships which, through complete adaption 

 to the parasitic habit, have arrived at a state where they 

 no longer possess a free living stage at any point in 

 their life cycle. In the reduction of organs character- 

 istic of free life the Acanthocephala represent the ex- 

 treme condition of complete absence of any structures 

 for locomotion in any stage of their development, and 

 total elimination of all special organs dealing with the 

 processes of metabolism. Similarly there has been an 

 entire loss of all organs of special sense. The whole 

 central nervous system consists of a small mass of gang- 

 lion cells from which a few fibers are distributed to the 

 body wall and to the muscles which control the opera- 

 tions of the proboscis and of the anterior region of the 

 body. In spite of the fact that these organisms are re- 

 duced to essentially little more than a sac for containing 

 the developing reproductive elements and a special 

 organ, the proboscis, for attachment to the host, yet they 

 display rather marked differences between the sexes in 

 many species. Many of these differences are apparently 

 of no advantage to the individuals possessing them and 



