CILIATA 



157 



fibrils which, by their contraction, withdraw the disc, and at the 

 same time circular fibrils close the peristome over it. In the 

 type-genus the pellicle is continued into a long, slender elastic 

 stalk (s), of which the longitudinal myoneme fibrils of the ecto- 

 plasm converge to the stalk, and are prolonged into it as a 

 spirally winding fibre, sometimes transversely striated. 1 The 

 effect of the contraction of this is to pull the stalk into a helicoid 



Fig, 60. Vorticella. A, expanded; B, stalk in contraction; c, eversible collar below 

 peristome ; cr, line of posterior ciliary ring ; c.v, contractile vacuole ; m, muscle 

 of stalk ; iV, meganucleus ; n, micronucleus ; p, pharynx ; r, reservoir of contractile 

 vacuole ; s, tubular stalk ; u.m, undulating membrane in vestibule ; v, hinder end 

 of vestibule. E 1 , E 2 , two stages in binary fission ; E 3 , free zooid, with posterior 

 wreath ; F 1 , F 2 division into mega- and micro-zooids (m) ; G 1 , G 2 , conjugation ; m, 

 microzooid. (Modified from Biitschli, from Parker and Haswell.) 



spiral (like a coil -spring), with the line of insertion of the 

 muscle along the inner side of the coils, which is, of course, the 

 shortest path from one end to the other (Fig. 60, B). 



The members of the Vorticellidae are very commonly attached 

 to weeds or to various aquatic Metazoa, each species being more 

 or less restricted in its haunts. Vorticella, the type, is singly 



1 Covered with a rather lax structureless membrane (sarcolemma), which is 

 spirally wrinkled when the muscle contracts. I am unable to verify Geza Entz*s 

 observations, adopted by Calkins and Delage. 



