88 ILLINOIS BIOLOGICAL MONOGRAPHS [88 



60-68; Taf. II, Fig. 18). It corresponds in detail to the system in Cercaria 

 gracillima previously described. 



Coupled with these morphological likenesses are the experimental data 

 of Leiper (1915, 1916), Miyuri and Suzuki (1914), and Iturbe and Gonzalez 

 (1917) on Schistosoma life-histories. Leiper has found that the three 

 schistosome species known to infect man, Schistosoma haematobium, S. man- 

 soni, and S. japonicum, give rise to miracidia which have a certain attraction 

 for certain snails in the districts infected. Within the tissues of these snails 

 the miracidia metamorphose into sporocysts, and the second generation sporo- 

 cysts give rise internally to bifid cercariae with a ventral sucker but without 

 a true pharynx. Furthermore, these cercariae introduced thru the skin of 

 experimental animals, mice and monkeys, give rise to typical unisexual adult 

 schistosomes. 



This two-fold evidence favors the view that the f urcocercous apharyngeal 

 distome cercariae, including Cercaria gracillima and C. tuberistoma of the Bitter 

 Root fauna, develop, under proper conditions, into adult schistosomes. One 

 character figured by Leiper for all of his apharyngeal bifid cercariae, yet not 

 used by him as a diagnostic character, is the presence of salivary-mucin glands. 

 On the other hand, one character used by Leiper for cercariae of the group, 

 namely, the absence of a pigment eye, holds for his three species of schistosome 

 larvae, but does not hold for Cercaria ocellata La Val., C. douthitti Cort, or C. 

 gracillima. It has been shown that the eye-spot may be present without 

 pigment (C. gracillima). Likewise the failure of Looss to mention an eye- 

 spot for C. vivax Sons. (1896:216-223; Figs. 172-174) does not necessarily 

 imply the absence*'of pigmentless eye-spots in that species. 



The relationship of the forked tail apharyngeal distome cercariae will 

 be made much clearer by a consideration of the genital organs. The adult Schis- 

 tosomas into which the larvae of Leiper develop are characterized by a relatively 

 small number of testicular follicles, 4 to 5 for S. haematobium, 6 to 8 for S. 

 japonicum, and 8 for S. mansoni. On the other hand, the testicle prolifera- 

 tion in Cercaria gracillima (Fig. 149) shows twenty-four or twenty-five fol- 

 licles already differentiated from a posterior germ mass. The large number 

 of testicular follicles has been found to be characteristic of the adult Schisto- 

 somatidae of the genera Bilharziella (Kowalewski,1895; 1896), Gigantobil- 

 harzia (Odhner, 1912), and Ornithobilharzia (Odhner, 1912). None of these 

 genera and in fact no Schistosomatidae have been reported from North 

 America. 



From these considerations it seems probable that Cercaria gracillima is 

 the larva of a schistosome genus, such as Bilharziella, Ornithobilharzia or 

 Gigantobilharzia, all of which are bird parasites. 



Since the germinal masses of Cercaria gracillima are hermaphroditic, and 

 any marked differentiation of the organs does not take place while the larva 

 is outside the definitive host, differentiation of sex in Schistosomatidae takes 

 place in these species comparatively late in their metamorphosis. This view 



