20 ILLINOIS BIOLOGICAL MONOGRAPHS [20 



that grow into the parthenogenetic individuals. In the simplest types the 

 germinal cell mass consists of the entire internal layer lying next to the ecto- 

 derm. Such a type is seen in Cercaria diaphana (Fig. 79), and \nC. micro- 

 pharynx (Fig. 94). In the majority of cases, however, the germinal tissue 

 is localized at one end of the sporocyst. In tvi^o cases at least there is the 

 differentiation of a muscular attachment organ at the antipodal end (C 

 dendritica, Fig. 87; C. racemosa, Fig. 105). In the furcocercariae, C. gracillima 

 and C. tuberistoma (Figs. 147, 157), there is a rhizoid-like attachment at the 

 germinal end. In these cases there seems to be some evidence for regarding 

 the germinal layer as localized at the end opposite the potential mouth. 



The redia is the type of the life cycle normally developing within the 

 sporocyst. Its organization is much more complex than that of the sporocyst. 

 There is a well-developed oral aperture, a muscular pharynx, and a sac-like gut. 

 There is a birth-pore just behind the collar region, on the left side, slightly ven- 

 tral. Two projections, usually in the posterior part of the body, readily differ- 

 entiate the redia externally from the shapeless sporocyst. With some justifica- 

 tion Ssinitzin (1911:76) regards these projections as comparable to an originally 

 bifid tail of the cercaria as in Bucephalus. In the cephahc region around the 

 pharynx there is a nerve complex of highly difi'erentiated nerve cells and nerve 

 fibers. These are distinguishable as a central nerve ring, with four anterior 

 and four posterior trunks. The posterior trunks do not develop far caudad. 

 The integument is well developed and thick, and muscular layers within it 

 play an important role in the movement of the redia, whereas the sporocyst 

 depends almost entirely for its movement on the motility of the larvae within 

 it. In the mature redia the germ tissue is always locaUzed at the posterior 

 extremity of the body. 



The development of the germinal tissue of sporocyst and redia has been 

 shown to be the result of the maturation of parthenogenetic eggs. The sig- 

 nificant correspondence between the localized germinal epithelium of the 

 parthenita and that of the cercaria may be pointed out here. In most cercariae 

 the male germ cells are aggregated into a definite number of testicular masses, 

 in most cases, two. In the apharyngeal furcocercariae (the probable larvae 

 of the Schistosomatidae) the number of germ masses is larger. The data 

 compiled in Table IV, on the better known Schistosomatidae, show that the 

 number of the testicular follicles varies from four to five in Schistosoma haema- 

 tobium, the mammaHan parasite (Looss, 1899:658) to about 134 in Bilharziella 

 polonica, the avian parasite. The origin of these testes is not described in 

 any case. In all of the adults the sexes are separate. In Cercaria gracillima 

 (Fig. 149) the testicular masses are proliferated from a germinal mass at the 

 posterior extremity of the body, ventral to the excretory bladder. They are 

 numerous; some twenty -four or twenty-five masses are found in this region at 

 this stage of maturity. Moreover, the female cell masses are also present in 

 the species at this larval stage, showing that the animal is not primitively 

 unisexual, but hermaphroditic. It would be only one step further back in 

 the phylogeny of the group to assume that the hermaphroditic cell masses and 



