34 ILLINOIS BIOLOGICAL MONOGRAPHS [314 



whereas they are absent in this species. In certain particulars P. or- 

 biculare resembles P. opacum, but the two species have different num- 

 bers of hooks in the genital coronets ; they differ also in the relative size 

 of caudal suckers. The great hooks of the caudal disc are present in 

 P. opacum. The two species differ also in that one is parasitic in the 

 urinary bladder and the other in the oral cavity. 



POLYSTOMA OPACUM Stunkard 1916 

 [Figures 15 to 21] 



Two worms of this species were obtained from the esophagus of a 

 single specimen of Trionyx ferox from Newton, Texas, and another 

 from the esophagus of Malacoclemmys lesueurii from the same region. 

 These trematodes were the same color as the lining of the esophagus 

 and so firmly attached that they were removed only with great difficulty. 



The worms (Fig. 15) measured 4, 3.75, and 3.25 mm. in length and 

 1, 0.85 and 0.8 mm. respectively in width. The body has an elongate 

 oval outline, is flattened dorso-ventrally, and observed in living condi- 

 tion, shows great variations in shape. In an extended condition it nar- 

 rows at either or both ends, and the contracted form may be not more 

 than half the length when extended, and broadly oval or quadrate in 

 outline. The caudal disc is slightly wider than the body in the mounted 

 specimens, measuring 1.09 and 1.21 mm. in width while each sucker is 

 approximately 0.4 mm. in diameter. The suckers have a chitinous skele- 

 tal framework, as is described in the generic discussion. In the external 

 meridinal band there are thirty-two divisions, which number corre- 

 sponds to the number of hooks in the genital coronet. The suckers are 

 arranged in a circle, altho the anterior pair are separated by a distance 

 slightly exceeding that between the posterior pair. Between the 

 anterior suckers there are many chitinous spicules, and in one specimen 

 two of the larval hooklets. Chitinous spicules are present on the sides 

 of all the suckers and over the ventral surface of the disc. Between the 

 posterior suckers there are three pairs of hooks, viz. two pairs of the 

 small larval hooks and one larger pair, but the great hooks are relatively 

 much smaller than the corresponding hooks in other species in which 

 they are present (Fig. 40). The larval hooklets are 7 to 9/a in length 

 and the great hooks are 75/a in length. The chitinous spicules present 

 on the disc have no definite arrangement and their points may stand 

 in any direction; the three larval hooks between the anterior suckers 

 of one specimen have no definite relative position and their hooks point 

 in different directions; those at the posterior edge of the disc are set 



