///. CILIATA: HJPOTRICHA. 



211 



spiral muscle. This extends into the body and divides up into fine fibrilla3 

 which extend under the cuticle to the peristome. When the muscle in the 

 stalk contracts it becomes coiled into a corkscrew spiral, drawing back the 

 animal, and folding in the anterior end. Vorticella* is solitary; Carche- 

 sium* forms colonies with dichotomously branched stalks; Zoothamnion* 

 colonies imbedded in a common jelly ; Epistylis* (fig. 147), branched col- 

 onies with rigid stalks, the muscle being confined to the base of the body. 



Order IV. Hypotricha. 



In this order the body is more or less flattened and a ventral 

 and a weakly arched dorsal surface are differentiated. The back 

 lacks cilia, but often bears spines and tactile bristles. On the ven- 

 tral side are several longitudinal rows of cilia, and besides straight 



ID 1 .. 



FIG. 151. FIG 152. 



FIG. 151. Stylonychfa mytilns. (After Stein.) , anal hooks; 6, ventral hooks; c, con- 

 tractile vacuole: rf, frontal ridge; g, canal leading to contractile vacuole; I, upper 

 lip; 71, nucleus with micronucleus: p, adoral ciliated spiral; r, marginal cilia; s, 

 caudal cilia; .sf, frontal spines; z, anus (cytopyge). 



FIG. 152. Division of Sti/ionychia mytilns. (After Stein ) c, c', contractile vacuoles of 

 the two individuals; n, ?V, nucleus and micronucleus; p, p', adoral ciliated spiral; 

 r, ', marginal cilia; w;, ;', ciliated ridges. 



spines and hooked cilia composed of united cilia. These latter are 

 of use in creeping. The strongly developed adoral cilia are of use 

 in locomotion and in producing vortices which bring food. The 

 macronucleus is often divided into two oval bodies connected by a 

 thread; the micronuclei vary in number from 2 to 4 in the same 



