PAPERS ON ZOOLOGY 285 



constant in all individuals. Consequently any difference 

 in body size, in members of this family at least, could re- 

 sult only from differences in physiological processes 

 which would pennit simple increase in bulk without any 

 corresponding increase in number of cells. Since the 

 start of this diff'erentiation in bulk of the members of the 

 opposite sexes occurs at about the same time that the 

 germ cells start to form it seems possible that simple 

 difference in size of the two sexes may be directly corre- 

 lated with differences in physiological conditions accom- 

 panying the development of the sex cells. 



General body form frequently shows marked contrast 

 in the two sexes other than the relative size discussed 

 above. In Arhi/thmorhifnchus pumiUrostris Van C. the 

 writer has shown that the male has the posterior region 

 of the body distinctly attenuated while the gravid female 

 displays no distinctive difference in diameter of anterior 

 and posterior regions of the body. In this species the 

 musculature of the body wall is also apparently more 

 highly developed in female than in male. In preserved 

 specimens the females present a distinctly wi'inkled ap- 

 pearance due to the contraction of the muscles in the 

 body wall, while the males present a perfectly smooth 

 surface on the exterior of the body. 



Frequently a portion of the body of a gravid female 

 becomes distorted from the form characteristic of the 

 young female and of the male. Localized distended 

 areas have frec[uently been attributed to the mechanical 

 effect of the myi'iads of developing embryos which fill 

 the entire body cavity of the gravid female. Thus in 

 Xeoechinorhi/nchus cylindratus (VanC.) and in X. agilis 

 (End.) the middle third of the body of the female us- 

 ually shows a distinct enlargement. In some instances 

 the entire body becomes greatly distended, forming a 

 capacious sac for the retention of the embryos as the 

 writer has described in the females of FilicoUis hofulus 

 VanC. 



Dift'erence in size and form are not restricted to the 

 body proper. A radical difference in form of the pro- 

 boscis has been described for the female of FilicoUis 



