10 THE PROTOZOA. 



A. General appearance. 



Vorticella consists of a body, somewhat conical in shape in 

 the expanded condition, and with its apex or proximal end 

 prolonged 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 sud- 

 denly coiled up into a corkscrew spiral. If the irritation be 

 discontinued, the stalk gradually straightens out, the disc and 

 peristome become everted, and the ciliary movement is re- 

 sumed. 



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 between 

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

 fluid mass, or endosarc. The stalk is a prolongation of the 

 ectosarc only. 



1. The body. 



a. The cuticle, 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 between 



the disc and the peristome : it marks tlie ' ventral ' 

 surface of the animal. 



