l34 S. GfOTO. 



than two consecutive sections (each = 0.01 mm). Dieckhoff ^ has 

 described in Polystoinum integerriimim a sort of valvuhir mechaniam at 

 the opening of the canal; I have not myself observed the canal 

 running any distance into the intestine, but have found it always to 

 contract to an exceedingly small size just before opening, and to 

 communicate with the intestine by a minute pore which will be at once 

 closed by pressure from within the intestine. 



The homology of the genito-intestinal canal will be discussed later 

 on under ' General Considerations.' 



Atriuji genitale — I shall adopt this name for the cavity into 

 which, in many genera, the vas deferens and the uterus, or else the 

 former alone, open, whether this cavity be large and open to 

 the exterior by a small pore, or merely a shallow invagination of the 

 general surface of the body. This cavity has been more generally 

 known as the genital cloaca; but since the vas deferens only in some 

 cases opens into it I prefer the above name for it. It is very generally 

 pi'esent not only in the Monogenea but also in the distomes; and in 

 some cases what has been described as the penis or " Girrusbeutel" 

 (tasca del pene, poche de cirrhe) is to be called more correctly the 

 genital atrium, as will become clear as we proceed. 



Among the species that I have hitherto examined the genital 

 atrium is absent only in Onchocotijle spinacis, in which the vas deferens 

 opens, as already stated, directly into the terminal part of the uterus 

 (PI. XVI, fig. 7). This part might perhaps be considered by some as 

 the genital atrium; but as may be seen from the figure just referred to, 

 its internal surface is covered up to its very opening with stout cilia 

 exactly similar to those of the part of the uterus posterior to it ; and 

 this fact should prove beyond doubt that the genital atrium is 

 totally wanting in this species. Moreover it seems to me that the 



1). DieckhofE— i. c p. 248. 



