295] XORTH AMERICAN POLYSTOMIDAE—STUNKARD 13 



same cavity, became fused at their internal ends with the vitelline ducts 

 and this union continued outward to the location where the vitelline 

 ducts turn toward the follicles and the vaginae branch off to open to the 

 exterior. The advantage of a single duet over two ducts lying side by 

 side is obvious, and the fusion of two parallel ducts is not uncommon in 

 other groups. With a further dorsal migration of the opening of the 

 vaginae there would be a separation of the vitelline and vaginal canals 

 and a dorsal fusion of the vaginae would give the single dorsal vagina of 

 Octobothrium, Axine, and Microcotyle. The earlier fusion of the vitel- 

 line and vaginal canals would retard the secondary fusion of the internal 

 ends of the dorsal vaginae and this explains the single dorsal pore and 

 internally paired vagina of Axine hetcrocerca which is used by Odhner 

 as an argument supporting his idea that in the Monogenea two different 

 structures are included under the term vagina. 



I agree with Odhner that the seminal receptacles of Sphyranura are 

 homologous to the paired vaginae of Polystoma, and that this furnishes 

 a splendid example of the change whereby the terminal part of the uterus 

 has assumed the copulatory function. It may be that further specializa- 

 tion in this direction, due to the endoparasitic habit and self fertilization, 

 may explain the absence of the vagina of the distomes. 



It now remains only to account for the absence of the genito-in- 

 testinal canal in the Monopisthocotylea. Odhner stated that this struc- 

 ture is homologous with Laurer's canal, and in his (1912) paper called 

 attention to the fact that Laurer's canal is a "rudimentary organ" 

 which serves no essential function. The vestigeal character of Laurer's 

 canal is believed in by most writers — Looss, Monticelli, Brandes, Goto, 

 etc. This structure is entirely lacking in some distome groups and in 

 others is represented by a blind sac opening from the ootype. Since the 

 genito-intestinal canal is admittedly homologous to Laurer's canal and 

 the latter is known to be a vestigeal structure, it appears reasonable to 

 suppose that it has degenerated in the Monopisthocotylea. 



There is a possibility that the Monopisthocotylea instead of having 

 lost a genito-intestinal canal may have arisen from a group of the Tur- 

 bellaria which had no homologous structure, but this explanation seems 

 very improbable. Haswell (1907) described in certain Australian poly- 

 clads a tube which formerly had been considered an accessory or dorsal 

 vagina but which in certain forms opened into the intestine. The pres- 

 ence of this genito-intestinal canal in polyclads, he says, ' ' strengthens the 

 contention, so ably supported by Goto, that the genito-intestinal canal 

 and not the vagina of the Heterocotylea is the equivalent of the Laurer 's 

 canal of the Malacocotylea." 



The absence of the genito-intestinal canal in the Monopisthocotylea 



