THE INFUSORIA 



367 



forms the mouth, though still in its primitive position at the anterior 

 end, is always open, and presents a clear passage from the exterior 

 to the medulla (Prorodon) (Figs. 8 and 9). In Torguatella (Figs. 12 

 and 66) there is a lip-like, supra-oral lobe projecting above the 

 mouth, and in Trachelius (Fig. 7), Spirostomum, and other genera, 

 there is a long, pointed prostomial process. It is impossible to 

 determine whether this prostomial process is rightly regarded as 

 an outgrowth in front of the mouth, or whether it is due to a shifting 

 backwards in the position of the mouth, but in such forms as 

 Paramoecium, and in the Hypotricha generally, the ventral position 

 of the mouth can only be explained on the supposition that, in 

 accordance with the method of feeding, it has shifted its position 

 from the primitive one at the anterior end of the body. 



In most of the Ciliata very little differentiation can be observed 

 in the protoplasm surrounding the oral aperture, but in a few cases 



Fio. 9. 



The horny fascicu- 

 late lining of the 

 mouth of Prorodon, 

 isolated. 



FIQ. 8. 



Prorodon nivens, 

 Khr. , one of the Holo- 

 tricha. a, nucleus ; b, 

 contractile vacuole ; 

 c, mouth with horny 

 fasciculate lining. 

 X 75. 



FIG. 10. 



Section through the mouth and 

 pharynx of Urotricha lagenula, Kent. 

 m, mouth ; Ph, pharynx ; c, cortex ; M, 

 medulla. (After Schawiakoff.) 



a series of rod-like thickenings of the cortical pro- 

 toplasm guard the mouth. In Prorodon (Fig. 8), 

 for example, there is a paling arrangement of such 

 rods which appears to serve the purpose of keeping the aperture 

 distended. In- Urotricha these rods make an apparatus of a more 

 complicated type sunk below the plane of the mouth, forming 

 what might be called a pharynx (Fig. 10). The mouth of the 

 Infusoria may be situated either at the surface of the cortex or 

 sunk to the bottom of a funnel-shaped depression usually called 

 the vestibule, and on the slopes of this or in the neighbourhood of 

 its margin many specialised differentiations of cilia or groups of 

 cilia may be found (the paroral cilia) adapted to the function of 

 driving the food into the mouth (Blepharocorys, Fig. 4). 



The mouth or the vestibule is sometimes overhung by one or 

 two membranous expansions of the cortex, the free edges of which 

 are provided with a special row of cilia. In Opercularia this 

 membrane is like a lid or operculum which closes the aperture of 

 the vestibule before the peristome is contracted. 



