392 Dr. R. Greef on the Structure and 



behind the peristome (PI. XVI. fig. 1, and PI. XV. fig. 5,//)'. 

 Fromt his aperture (PI. XVI. figs. 1 & 2, m ff) a tolerably 

 wide canal runs inwards directly behind the ciliated disk and 

 parallel to its plane ; it then makes a knee-like bend (PI. XVI. 

 fig. 1, h) and, gradually becoming narrower, runs again to 

 the buccal (ventral) side and then backwards, describing in 

 its course two or three more slight curves. As far as the 

 knee-like bend, according to Lachmann, the initial part of the 

 alimentary canal, the so-called vestibulum, extends. At the 

 bottom of the cavity of this knee, behind a long bristle which 

 traverses the whole initial portion of the alimentary canal and 

 projects from the external aperture (PI. XVI. fig. 2^ff) the 

 anus is situated (fig. 2, h) ; and here, therefore, according to 

 Lachmann, the mouth should begin. 



This determination of the buccal and anal apertures and of 

 the initial portion of the alimentary tube as the vestibulum, 

 has certainly some justice on its side ; but in my opinion it 

 would render the whole conception of the alimentary canal 

 simpler if we were to retain the designation mouth for the 

 external aperture. That the anus opens into this initial por- 

 tion of the alimentary canal, and that consequently the ex- 

 ternal aperture serves at once for the introduction of food 

 and for the evacuation of exhausted materials, need not pre- 

 vent our designating it as the buccal aperture. We find pre- 

 cisely similar conditions in all Coelenterata and in many Echi- 

 noderms and worms, in which a special anus opening out- 

 wardly cannot be detected. Must we not in this case, for ex- 

 ample, characterize the alimentary sac of the Anthozoa also 

 as a vestibulum, and place the aperture of the mouth at its 

 open posterior end, as here also the true orifice of ejection 

 begins (tha.t is to say, it opens into this initial part of the ali- 

 mentary canal) ? Moreover, if we admit in designation and 

 conception that this vestibulum is to be regarded as the initial 

 part of the alimentary canal^ we need not hesitate to accept 

 the beginning of this initial part, ^. e. its external aperture, as 

 the buccal ajjerture. At any rate by this means the whole 

 conception of the alimentary canal, and especially its descrip- 

 tion and terminology, would be materially facilitated and sim- 

 plified. Lachmann's vestibulum would then, as formerly, be 

 designated ihe, 2Jharynx ; and in this we are justified by a pe- 

 culiar arrangement which exists in the kneed cavity of the 

 end of this pharynx. Thus we see distinctly that the particles 

 of food, driven into the mouth by the ciliary current, first of all 

 reach this point, and are then carried either back again and 

 out through the mouth or into the following section of the 

 alimentary canal. To eifect this there are, in Epistylis Jlavi- 



