196 



PROTOZOA 



Order II. Heterotricha. 



Like the Holotricha the Heterotricha are everywhere ciliated, but they have 

 a tract of stronger cilia, the adoral ciliated spiral, beginning at some distance 

 from the cytostome and leading in a spiral course into the mouth. It consists 

 of rows of cilia united into membranellce placed at right angles to the course of 

 the spiral. In the Stentors* (fig. 153), the peristomial area, surrounded by 

 the spiral, forms the broader end of the body, which tapers toward the other 

 end, by which the animal may attach itself. Muscle fibres running lengthwise 

 immediately under the cuticle produce energetic movements. Balantidiiim 

 coli (fig. 154) appears in the large intestine of men ill with diarrhoea, it also occurs 

 in swine without causing sickness. Other parasites of man are B. minutnm 

 and Nyctotherus faba. 



Order III. Peritricha. 



The Peritricha have a broad peristome area around the cytostome; the oppo- 

 site end has a corresponding pedal disc or is narrowed like a goblet and ends in a 

 stalk (fig. 155). Only the adoral ciliated spiral is constant. It arises from the 

 swollen margin T)f the peristomial area, and continues on the 'operculum,' a 



FIG. 155. Carehesium polypinum (after Butschli). Left, a single animal; right, 

 three stages of division, cv, contractile vacuole; n, macronucleus; n', micronucleus ; 

 Nv, food vacuoles; os, cytopharynx; per, peristome; 7-5, reservoir of contractile vacuole; 

 urn, undulating membrane; vst~ vestibule; wk, ring on which a posterior circle of cilia 

 may develop. 



disc which projects free from the peristomial area, but in contraction is 

 close against it, the peristome lips folding over all. Besides, there may 



ciliated 



drawn close 



be a temporary or permanent circle of cilia near the hinder end. The nucleus 



is usually sausage-shaped, much bent, and with the small micronucleus in its 



hinder angle (fig. 155, n'). 



The VoRTiCELLiD.(figs. 152, 155), are attached by a long stalk which 

 contains a slightly spiral muscle, dividing in the body into fine fibrillae which 

 extend under the cuticle to the peristome. When the muscle in the stalk con- 

 tracts it becomes coiled into a corkscrew spiral, drawing back the animal, and 



