10 THE PROTOZOA. 



of the weed or other substance to which it is attached. Examine 



it microscopically. 



A. General Appearance. 



Vorticella consists of a body, somewhat conical in shape in 

 the expanded condition, and with its apex or proximal end pro- 

 longed into the slender stalk by which the animal is attached. 



The distal end of the body, or disc, forming the base of 

 the cone, is bordered by a prominent rim, the peristome. 



The various species of Vorticella differ somewhat in form 

 and proportions from one another. 



Vorticella is highly irritable, and when disturbed, as by 

 lightly touching the cover-glass with a needle, it instantly 

 contracts. The peristome is suddenly turned in, concealing the 

 disc and cilia, and at the same moment the stalk is suddenly 

 coiled up into a corkscrew spiral. If the irritation be discon- 

 tinued, the stalk gradually straightens out, the disc and peri- 

 stome become everted, and the ciliary movement is resumed. 

 B. Structure. 



Vorticella is a protozoon, and the whole of the animal, 

 stalk and all, is one single cell, comparable to a single Amoeba 

 or Paramecium. The entire animal consists of protoplasm, 

 in which, as in Paramecium, a distinction may be made be- 

 tween the outer and firmer layer, or ectosarc, and the central 

 and more fluid mass, or endosarc. The stalk is a prolongation 

 of the ectosarc only. 

 1. The body. 



a. The pellicle, or outermost layer of the ectosarc, forms 



a protective investment to the whole body. 



b. The disc is the distal or free end of the animal : it 



is slightly convex. 



c. The peristome is the projecting lip or rim surround- 



ing the disc, and separated from it by a circular 

 groove. 



d. The vestibule is a conical pit-like depression be- 



tween the disc and the peristome : it marks the 

 1 ventral ' surface of the animal. 



e. The pharynx is a narrow continuation of the inner 



