DIGESTION IN CILIATES 263 



in length, and evenly distributed. Their function 

 has become locomotory only. A further simplifica- 

 tion is seen in that Opalina has no mouth and no 

 anus. Its nourishment is obtained by absorbing 

 fluids around it by its entire body surface. 



The cell gullet, or cytopharynx (Fig. 49, g), is 

 large in free-living Ciliates, and is reduced in para- 

 sitic ones. A cell anus is present in some cases, but 

 it is remarkable that in some of these organisms 

 occurring in cattle there is a well-marked anus, and 

 yet no mouth is present. In many Ciliates the 

 intake of food is accomplished at a cytostome. 

 Various food particles are surrounded by small 

 quantities of fluid forming food vacuoles, and the 

 vacuoles circulate through the body in a definite 

 direction. When all the nourishment has been ab- 

 sorbed, each particle of debris is diverted from the 

 circulating area and pressed from the body at any 

 point if no cytopyge is present, or passes from the 

 regular channel if such is available. In a small 

 Ciliate parasitic in pond snails we have been able 

 to watch the intake and digestion of food and the 

 expulsion of the waste matter, the latter occurring 

 through the mouth, a most unusual circumstance. 



The two nuclei of Ciliates have already been men- 

 tioned. The macronucleus is usually very large, and 

 frequently is curved. In some organisms (Spiro- 

 stomum, Stentor) the macronucleus is beadlike. Altera- 

 tions in the shape of the nucleus occur in some 

 species, and it can move from one part of the body 

 to another. The micronucleus, when present, is 

 small, and divides during reproduction. In some 



