92 ILLINOIS BIOLOGICAL MONOGRAPHS [92 



each case one or more ganglion cells fill the optic cup. In each it can be 

 traced to the brain center. Parasitism has caused the pigment eye to degen- 

 erate in most adult Digenea. Even in the cercariae it is pigmented only in 

 certain species; other species have lost all traces of pigment, but the optic 

 nerve is still present and can be traced to the brain center. In many cases 

 it has been lost, even in the cercariae. Eye-spots are common in miracidia, 

 but are lost on metamorphosis into the sporocyst. There are no records of 

 eyes in rediae. 



5. The digestive tract of the cercariae and the adult hermaphroditic genera- 

 tions of trematodes is usually triclad. In the redia the gut is rhabdocoel; 

 there is a pharynx and saUvary glands have been described (Ssinitzin, 1911, 

 and pp. 63, this paper). In the miracidium, there is an indication of a 

 rhabdocoel gut and salivary glands (Looss, 1892; Miyuri and Suzuki, 1914), 

 altho these are usually lost in the adult. In the Turbellaria the gut is triclad, 

 polyclad, rhabdocoel or acoel. The pharynx is ordinarily present and salivary 

 glands are common. 



6. The excretory system in the two groups is at first a single pair of pro- 

 tonephridia. This condition is found in miracidia, rediae, cercariae and Tur- 

 bellaria alike. The capillaries, whether in the larva or adult, end in flame 

 cells. 



7. The genital system was one of the first in which investigators recog- 

 nized the relationship between the Trematoda and the Turbellaria. The 

 hermaphroditic condition in the hermaphroditic generation of the Trematoda is 

 strikingly similar to that of the Turbellaria, with the common genital atrium 

 in all groups except Acoela. Parthenogenesis in parthenitae is the result 

 of the great change in environment of these cycles. 



Thus the main trend of investigation has come to support the common 

 origin of the Trematoda and the Turbellaria. 



LIFE CYCLE OF THE DIGENETIC TREMATODES 



The life-history of the trematode of the order Digenea consists not in an 

 alternation of sexual and asexual generations, but rather of successive sexual 

 generations which are parthenogenetic and hermaphroditic. 



A problem which has arisen in connection with the genitaUa of the Digenea 

 is the significance of Laurer's canal. Looss (1893a) considers it homologous 

 to the uterus of Cestoda, while Goto (1893) beHeves it to be the homolog of 

 the blind vagina of AmphiUna and the genito-intestinal canal of Monogenea. 

 According to Liihe (1909) a Laurer's canal is present in Monostomes, and in 

 some groups of the distomes, including Plagiorchiidae and Echinostomidae. 

 It has not been recorded for holostomes or Schistosomatidae. Except for the 

 Echinostomid cercariae, a Laurer's canal has been found in all cercariae of 

 the groups studied by the writer, where the adult trematode has the canal. 

 Absence of the canal in Echinostomid cercariae can be explained on the 

 basis of late development of the genital organs in this family. On the other 



