THE INFUSORIA 



397 



extremity of the body. In several forms the passage is kept open for the 

 greater part of its length by a palisade arrangement of stiff rods commonly 

 called the pharynx, but the mouth itself can be closed by the cortex 

 contracting over the anterior end of the pharynx (Prorodon, Coleps). 



The position of the mouth is by no means con- 

 stant in the group. In Spathidium (Fig. 45) it is 

 somewhat elongated, and situated just behind the 

 anterior end of the body. 



In Nassula it is situated at a distance of about one- 

 third of the length from the pointed anterior end of 

 the body. 



In Dileptus and Tradielius (Fig. 7) there is a long 

 narrow lobe in front of the mouth. 



The cilia are, in the most primitive forms (Holo- 

 phrya, etc.), evenly distributed over the surfaces of the 

 body j in some of these, however, the cilia which are .d^ 



arranged in a circlet round the mouth are a trifle like mouth above.tnenu- 



i j ,1 . i / r . \ , , , i_ , i dens in the centre, and 



longer and thicker (Lacrymana) than those on the the contractile vacuole 



general surface. In the larger forms it may be ob- ^an (AfterButsc 111 '-) 

 served that the cilia are arranged in parallel rows 

 running longitudinally down the body or slightly curved like the rifling 

 of a firearm. In Didinium the cilia are confined to two fine horizontal 



Fio. 45. 

 Spathidium lieber- 



Fio. 46. 



1, Surface view of a Holotrichous Ciliate, showing the disposition of the cilia in longitudinal 

 rows. 2, diagrammatic optical section of the same, showing all structures except the contractile 

 vacuoles ; a, meganucleus ; b, micronucleus ; c, cortex ; D, pellicle ; E, medulla ; /, cilia ; 

 g, trichocysts ; h, filaments ejected from the trichocysts ; i, mouth ; k, drop of water contain- 

 ing food particles about to sink into the medulla and form a food vacuole ; I, m, n, o, food 

 vacuoles, the successive order of their formation corresponding to the alphabetical sequence of 

 the letters ; p, pharynx. 3, outline of a Paramoecium to show the form and position of the 

 contractile vacuoles. 4-7, successive stages in the formation of the contractile vacuoles. 

 (From Lankester.) 



bands, and in the parasitic Biitschlia and others the cilia occur in irregular 

 ridges and tufts. 



