30 ILLINOIS BIOLOGICAL MONOGRAPHS [310 



uterus. In the Polystomidae however, there is a definite specialized tube 

 leading from the ootype to the uterus. This duet is not homologous with 

 the oviduct, it is separated from that duct by the ootype, and further, 

 in the specimens examined by the writer the histological character of the 

 two are not precisely the same. The epithelial lining of the oviduct is of 

 the flattened type, and that of the second duct more columnar. Such a 

 duct is present in many cestode genera and is called the uterine duct. 

 The same name is proposed for the tube leading from the ootype to the 

 uterus in the Polystomidae, altho with the understanding that its use is 

 independent of the question of homologies of the female ducts in trema- 

 todes and cestodes. 



In P. bulliense the uterine duet opens into the uterus not at the end 

 but on the side, and there is a posterior uterine pocket. The uterus ex- 

 tends as a wide elongated sac from the extreme posterior end of the body 

 to the common genital sinus. In P. alluaudi the intracecal area is occu- 

 pied by the uterus and eggs are present almost as far posteriad as the 

 caudal union of the ceca. In P. integerrimum there is a long uterus 

 which extends in many loops anterior to the ootype, and contains a large 

 number of eggs. In all other known forms, the uterus is situated at the 

 level of the ovary on the opposite side of the body, and contains a single 

 large egg or embryo. Zeller (1876) described a similar condition for 

 the ectoparasitic form of P. integerrimum. Figure 14 shows a very early 

 embyro of P. orbiculare and Figure 23 a much later stage of development 

 in P. megacotyle. No shell is present in the former case, altho it may 

 have been lost in sectioning. There must be some provision for the 

 growth of the embryo and the shell can not be rigid during the uterine 

 period. Where the oviduct arises from the ovary, at its union with the 

 ootype, and at either end of the uterine expansion sphincter muscles pro- 

 duce short contracted portions of the tube. In all the species studied by 

 the writer, with the exception of the vitelline tubules, all ducts of the 

 female system have a fibro-membranous wall and an epithelial lining, 

 which in the ootype, uterine duct, and uterus consists of tall columnar 

 cells with distinct boundries and single nuclei. Describing the epithelial 

 cells lining the ootype in certain other monogenetic forms Goto (1894) 

 says that because of their appearance and reaction to stains he strongly 

 suspects their glandular nature, but since a shell gland is present he can 

 not understand their function. In certain species of Polystoma Mehlis' 

 gland is much reduced or absent, and in these forms the cells of the 

 epithelial lining of the ootype appear to be secretive (Figs. 8, 9). This 

 agrees with the present conception that the vitellaria secrete the shell 

 substance and Mehlis' gland the fluid in which the eggs are suspended. 



The genital pore is situated on the ventral surface in the median 



