III. CILIATA: HOLOTRICHA, HETEROTRICHA. 



209 



groups of markedly smaller microspores. The latter separate and 

 fuse completely with the macrospores, only a small cuticular sac 

 persisting to indicate the fusion. The nuclear phenomena are 

 much the same as with Paramcecium, allowance being made for 

 the permanance of the fusion. 



Order I. Holotricha. 



The Holotricha are doubtless the most primitive Ciliates, since 

 the cilia on all parts of the body are similar; being at most slightly 

 stronger at one end of the body or on the inside of the cytostome. 

 Best known are the species of Paramcecium* (fig. 144) occurring 

 in stagnant water. Opalina ranarum * lives in the intestine of the 

 frog. It lacks mouth, has numerous similar nuclei, no micronu- 

 oleus and no conjugation. The small encysted Opalines pass out 

 with the faeces, and are eaten by the tadpoles, which thus become 

 infected. 



Order II. Heterotricha. 



Like the Holotricha the Heterotricha 

 are everywhere ciliated, but they have a 

 tract of stronger cilia, the adoral ciliated 

 spiral. This is a band of cilia beginning 

 at some distance from the cytostome and 

 leading in a spiral course into the mouth. 

 It consists of rows of cilia united into 

 ' membranellse ' placed at right angles to 

 the course of the spiral. In the best- 

 known heterotrichans, the Stentors * (fig. 

 148), the peristomial area, surrounded by 



FIG. 148. FIG. 149. 



FIG. 148. Stentor polymorphus. (After Stein.) a, peristomial area; b, roof of hypo- 



storae; 0, contractile yacuole; n, nucleus; o, cytostome; r, adoral ciliated spiral; 



t, hypostome (excavation for mouth). 

 FIG. UU.Balantidiuni coli. (After Leuckart.) 



