134 



s. GOTO. 



than two consecutive sections (eacli=^0.01 mm). Dieckhoff'Mias 

 described in Pohjstomum iiiteijeryimwn a sort of valvular mechanism 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.' 



Atrium genitale — I shall ado})t this name for the cavity into 

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

 former alone, open, whether this ca^ity be large and open to 

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

 general surface of the b^dy. 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 

 present not only in the Monogenea but a.lso in the distomes; and in 

 some cases what has been described as the penis or *' Cirrusbeutel" 

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

 genital atrium, as will bec(^me clear as we proceed. 



Among the species that I have hitherto examined the genital 

 atrium is absent only in Onclwcotijlc ,spinacis, in which tlie ^'as deferens 

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

 (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 posteri(3r to it ; and 

 this fact should prove bevond doubt that the genital atrium is 

 totally wanting in this species. Moreover it seems t(3 me that the 



1). Dieckhofi— /. c. p. 2i8. 



