338 Ciliophora 



through the cytostome into the "vestibule" (pharynx). The majority are 

 sessile and are commonly equipped with stalks. 



The Chonotrichida are ectocommensals attached to their hosts by a 

 basal disc or a short stalk. The peristome, at the distal pole, is usually 

 surrounded by a funnel-like prolongation of the body, or sometimes by 

 two concentric funnels. The wall of the funnel may or may not be rolled 

 into a spiral. Cilia may be restricted to the peristome and funnel in the 

 adult stage. Reproduction by budding is characteristic. 



Order 1. Holotrichida. This large order, usually considered more primi- 

 tive than the rest of the Euciliatia, shows considerable diversification of 

 the peristomial area and in one group the cytostome has disappeared. 

 Such specializations furnish a basis for dividing the Holotrichida into 

 suborders. 



Suborder 1. Gymnostomina. The cytostome opens directly at the sur- 

 face or else into a slight depression, or oral groove, which lacks a well 

 developed peristomial ciliature. In many genera the cytostome lies at or 

 near the anterior end of the body. In others, the mouth has shifted 

 posteriorly to either a compressed or a broad flattened oral ("ventral") 

 surface. 



Suborder 2. Trichostomina. The cytostome usually lies on the ventral 

 surface at the base of a rather well defined oral groove, typically equipped 

 with one or more fields of densely set cilia. Fusion of peristomial cilia 

 into simple membranes or membranelles, or both, occurs in a few species. 



Suborder 3. Hymenostomina. The peristomial ciliature has become 

 modified into several membranes, perhaps derived phylogenetically from 

 the peristomial cilia of Trichostomina. 



Suborder 4. Thigmotrichina. The most characteristic feature of these 

 commensals is an anterior group of thigmotactic cilia serving for attach- 

 ment to the host. The cytostome is shifted to a position at or near the 

 posterior end of the body. In some families there is an anterior sucker, 

 a new organelle. 



Suborder 5. Apostomina. The ventral cytostome is so reduced in size 

 that ingestion is probably limited to very small particles. Beneath the 

 cytostome there is a pecidiar "rosette" (Fig. 7. 25, A-F) of uncertain func- 

 tion. The somatic ciliation includes less than 22 complete rows of cilia. 

 Life-cycles are often fairly complex. 



Suborder 6. Astomina. These are endoparasitic holotrichs without a 

 cytostome. The body is rather uniformly ciliated as a rule, but there may 

 be a small cilia-free area at the anterior end. 



Suborder 1. Gymnostomina 



Form of the body varies considerably. Ovoid, pear-shaped, spindle- 

 shaped, and long vermiform types are common, and laterally compressed 

 and dorso-ventrally compressed species are not unusual. The ciliation is 



