34 ILLINOIS BIOLOGICAL MONOGRAPHS [252 



ever, since the number of specimens here is small the writer is inclined 

 to attach relatively little importance to this feature. 



The life history of this group is relatively unknown, although the ear- 

 liest record of a monostome larva is said to date from 1817. An early 

 account is found in Filippi 1859. This author obtained from Bythinia 

 tentaculata (L) [= Paludina impure of Filippi] a larva he named Cerceria 

 lophocerca which he described (1-859:5) as follows: "Elle est caracterisee 

 par un bulbe pharyngien assex fort, par la presence de deux yeux ou taches 

 pigmentaires semilunaires avec une petite lentille dans la concavite, et 

 par une queue munie d'une crete membraneuse longitudinale. Dans l'in- 

 teneur du corps on voit des rudiments d'organes sexuels sous la forme de 

 trois masses vesiculates." 



According to Liihe (1909) the determination of Filippi is doubtful 

 since the description and figures show a close resemblance to Cercaria 

 fulvo punctata Ercol. which is an undoubted distome larva. Cort (1915) 

 on the other hand recognized distinct features in this larva and stated 

 that it is "entirely different from all other monostomes known." As such 

 it stands alone and unidentified. 



While the description of Filippi is meager and bereft of many diagnos- 

 tic characters a few outstanding features point to its alliance to this group. 

 This relationship is shown by the absence or at most only poorly devel- 

 oped oral sucker (Filippi, pi. I, fig. 3), by the presence of a strongly devel- 

 oped pharynx, and by the position of the three "rudimentary" sex organs. 



Key to species of Cyclocoelum 



1 (6) Uterus restricted to the intercecal zone 2 



2(5) Testes unequal in size 3 



3(4) Pharynx larger than oral sucker; ratio of posterior testis to 



ovary 2:1 Cyclocoelum mutabile (Zed.) 



4(3) Oral sucker and pharynx small, approximately equal in size ; 



ratio of anterior testis to ovary 4:3 



Cyclocoelum cuneatum nov. spec. 



5(2) Testes equal in size. Oral sucker larger than pharynx; ratio 



of testes too vary 3:1 Cyclocoelum leidyi nov. spec. 



6(1) Uterus not restricted to intercecal zone 8 



7(26) Uterus folding around the crura both dorsally and ventrally . . . 9 



8(21) Gental glands separated by uterine loops 10 



9(16) Testes equal in size 11 



10(13) Pharynx larger than sucker; vitellaria extending laterally 



beyond medial wall of crura 12 



11(12) Ratio of testes to ovary 5:2 



Cyclocoelum pseudomicrostomum nov. spec. 



12(11) Ratio of testes to ovary 4:3. . .Cyclocoelum microstomum (Crepl.) 



