Life-history and Anatomy of the Appendiculate Distoraes. 377 



points out it is, strictly speaking, the pars prostatica alone. Juel 

 thinks that the papillae may be of use in the act of copulation, but 

 in the young animal I am certain this is not the case. The penis 

 consists here of a portion of the genital vestibule, as the forward 

 portion of that organ can be protruded (PI. 25, Fig. 2, ^), while the 

 pars prostatica cannot be protruded. 



The genital vestibule (vestibulum genitale commune) is the name 

 given to the terminal portion of the genital tract of appendiculate 

 Distomes by Juel (1. c.) who took the term from Levinsen (1881). 

 It is a long, thick walled, canal which, according to the observations 

 of Juel, Monticelli (1891), and Looss (1896), connects the anterior 

 ends of both the ejaculatory duct and the uterus with the genital 

 opening: it furnishes, thus, an outlet for both male and female re- 

 productive products. Its structure in the youthful worm under dis- 

 cussion differs somewhat from that of the adult ApoUema appendicu- 

 laium as described by Juel. It is a long tube-like structure with a 

 diameter of 0.015 mm up to within a short distance (0.012 mm) of 

 the genital opening, where its diameter drops suddenly to 0.005 mm 

 (PI. 27, Fig. 12, gen. vest). Its lumen is extremely fine and is lined 

 with a smooth, delicate, intima. This is surrounded by a layer of 

 thick, circular muscles, surrounding which is a layer of longitudinal 

 fibres; outside of these is a layer of parenchyma in which are present 

 small, deeply staining nuclei ; and this is bounded on the outside by a 

 layer of circular and longitudinal muscle-fibres (PI. 27, Figs. 12, 13, 14). 

 At a distance of 0.012 mm from the genital opening the outer layers 

 of muscle-fibres and the parenchymatous layer cease and the walls of 

 the tube consist of simply the intima and the fibres immediately sur- 

 rounding it from this point to its termination at the genital opening. 

 (PI. 27, Fig. 12.) 



This terminal portion of the vestibule, it seems to me, has arisen 

 as an invagination of the body-wall and is homologous to the genital 

 cloaca or sinus genitalis of other Trematodes: Juel (1890, p. 28) is 

 also of the same opinion. The remainder of the vestibule, with its 

 complicated structure, probably represents the fused terminal portions 

 of the ejaculatory duct and uterus. Juel (1. c.) also expresses the same 

 opinion; he thinks, however, that the fusion has taken place, not by 

 the union of the two ducts, but by the dropping back of the end of 

 the ejaculatory duct from the genital cloaca, where we might expect 

 to find it, to the position representcMl by the forward end of the pars 



Zool. Jahrb. XI. Abth. f. Morph. 25 



