PAPERS ON ZOOLOGY 283 



in some instances appear after the single necessary copu- 

 lation so they can have no very essential relation to the 

 perpetuation of the species. All evidence seems to point 

 to the fact that the Acanthocephala are, as a gi'oup, too 

 far removed from any free living ancestors to make it 

 possible that dimorphism could have been carried over as 

 an inheritance from free living ancestral forms. It then 

 becomes a matter of some interest to examine these 

 instances of sexual dimorphism ui the hopes of finding 

 some of the factors responsible for their development. 



The somatic differences between the sexes encountered 

 in these parasites constitute two fairly natural classes 

 (a) differences in form and size, and (b) presence in one 

 sex of structures entirely wanting in the other. Under 

 the first of these are included all the differences in body 

 form, in body size, and in proportions of the body or of 

 any of its individual structures while the second, of much 

 rarer occurrence, is possibly based upon incomplete and 

 faulty observations. Frequently structures such as body 

 spines, apparently wanting in one sex, have, upon closer 

 examination, been found gi-eatly reduced in size or ob- 

 scured by other structures. 



Almost invariably the mature female acanthocephalan 

 is larger than the male of the same species. In some in- 

 stances, however, the difference is so slight that among 

 fully mature sjDecimens some males are as large as the 

 smaller females and in a few instances there is practi- 

 cally no external means of differentiating the sexes. This 

 last mentioned condition is best exemplified in Plag- 

 iorhynchus fonnosus VanC. as shoT\Ti in figures 1 and 2. 



Extreme differences in size are to be noted in Giganto- 

 rhyuclius liirudinaceus (Pall.) from the hog. The female 

 of this species may reach a length of 65 cm. while the 

 male rarely attains a length of more than 10 cm. In 

 average, mature, individuals the male is about 4 mm. in 

 diameter while the female measures about 6 mm. Thus 

 in this species the difference in length is much more con- 

 spicuous than the difference m diameter. Simple sexual 

 difference in length appears rather late in the develop- 

 ment of the individuals of most species that have been 



