91] LIFE HISTOR Y OF TREMA TODES—FA UST 91 



monograph on Amphistomum subclavatum, Looss (1892:156, 157) com- 

 pared the proHferation of the germ-balls from the body wall of the parthenita 

 to the production of eggs and spermatozoa in the marine polychaetes, and 

 suggested that other embryonic structures of the trematode were comparable 

 to annelid structures, so that they might be considered of phylogenetic value. 

 Recently Ssinitzin (1911:86) has spoken of the resemblance of the Trematoda 

 to Arthropoda and Trochelminthes, in view of the absence of any ciliary integ- 

 ument, in place of which, he says, the cuticula and the external skeleton 

 develop. 



During this time the theory of the common descent of the Trematoda, 

 Cestoda and Turbellaria has been gaining ground, not because they are all 

 "flatworms, " as Ssinitzin insists, but on the homologies of the genital, excretory 

 and nervous systems. The observations of Leuckart (1886:140), Schulze 

 (1853:178-195), and Schneider (1864:590-597) all support this view, while 

 Leuckart even saw the analogy between the gutless sporocyst and the Acoela. 

 Lang (1884:669) showed that the nervous system of the Trematoda and 

 Turbellaria was homologous. 



With the work of Haswell on Temnocephala (1888) the close affinities 

 of the Trematoda and the Turbellaria became evident. In this group the 

 excretory system, the three anterior and posterior nerve trunks, the anterior 

 mouth, and the dorsal pigment eyes — all these bridged the way for the ac- 

 ceptance of the thesis that the Trematoda and the Turbellaria have a common 

 ancestry. 



The study of the parthenogenetic generations of the Digenea, to which 

 this paper is devoted, stands in support of this thesis and contributes the 

 following facts towards its further acceptance. 



1. The body cavity of the hermaphroditic generations of trematodes and 

 of Turbellaria is filled with differentiated mesenchyme and connective tissue. 

 In the parthenitae the parenchyma is confined to the body wall because it is 

 less diJGferentiated. 



2. Both Trematoda and Turbellaria are typically flat, with a pronounced 

 bilateral symmetry. The cylindrical appearance of the parthenitae is a 

 secondary modification due to parasitism. 



3. The epidermis of Trematoda and Turbellaria consists of a single layer 

 of cells. In the sporocyst larva, the miracidium, and in the Turbellaria, the 

 epithelial layer is ciliated. In the rediae and in the cercariae the layer is 

 usually sloughed off before maturity and in its place the basement membrane of 

 mesodermal origin, serves as the integument. 



4. The nervous system of the Monogenea, the Digenea, and the Turbellaria 

 is reducible to a common type. Two brain gangha with a transverse commis- 

 sure, three anterior pairs of nerve trunks, and three anterior pairs of nerve 

 trunks — these are common to all three groups. Moreover, the pigment eye- 

 spots of the Turbellaria, Monogenea, and Digenea are not only homologous, 

 but practically identical in detail (Hesse 1897; and page 52 this paper). In 



