308 Illinois State Laboratory of Natural History. 



provided with a number of rods called trichocysts. These 

 are used for defense, and are comparable with the nemato- 

 cysts in Hydra. 



The suborder Spirotricha, which includes the remaining 

 ciliates, is characterized by the presence of a spiral or nearly 

 circular wreath of cilia — the adoral wreath — leading to the 

 mouth opening, and partly or entirely enclosing a usually 

 well-marked area known as the peristome-field. This sub- 

 order comprises four quite well-marked divisions, Heterotri- 

 cha, Oligotricha, Hypotricha, and Peritricha, distinguished 

 largely by the ciliary structure and distribution. 



The Heterotricha have the body clothed with short, fine 

 cilia and an adoral circle or spiral of longer cirrose cilia at 

 the anteiior end, around the peristome-field. Stentor polymor- 

 phus may be taken as typical of this group. The body is 

 variable in form, but may be described as trumpet shaped, 

 expanded anteriorly and attenuated posteriorly. It is some- 

 times found in large attached colonies, and at other times 

 is free-swimming. The body is covered with longitudinal 

 rows of very fine cilia, while those around the peristome are 

 modified into strong flattened structures. The left margin of 

 the peristome is involuted, forming a little pocket, at the 

 bottom of which is the oral opening. The endoplasm usu- 

 ally contains a number of granules. The ectoplasm is 

 supplied with a layer of fine longitudinal contractile fibrils, 

 called the myophan. The contractile vacuole is large and 

 the nucleus large and moniliform. 



The Oligotricha are a rather small group of short, rounded 

 to obconic forms, with the peristome-field occupying the 

 the anterior end, as in Stentor, and surrounded by a nearly 

 or quite completely circular adoral wreath. The ciliation of 

 the body varies in amount. A lorica is sometimes present. 



The Hypotricha differ from the other groups in that they 

 are usually flattened and have the locomotor cilia upon the 

 ventral surface. These cilia are frequently modified into 

 strong styles or uncini. The dorsal surface is smooth or 

 furnished with a few rows of stiff cilia. The oral and anal 

 openings are distinct. Usually the peristome-field is on the 



