Protozoa and Rotifera at Havana, III. 305 



nucleus. One or more contractile vacuoles are usually 

 present except in the Sporozoa and Cystoflagellata, in which 

 none have as yet been discovered. 



As a pool dries up, the Protozoa it contains assume a 

 spherical shape and secrete about themselves a chitinous shell, 

 when they are said to be encysted, the spheres being called 

 cysts. In this condition they can readily withstand drought, 

 and when rain comes and fills up the pool they revive, 

 break through the chitinous envelope, and assume their former 

 shape. Encystment may also take place just before spore 

 formation. 



Protozoa occur abundantly in every pond or wayside pool 

 as well as in the larger bodies of water, and one might 

 naturally think that they would be favorites with the zoologists 

 and be thoroughly well studied ; but in the United States 

 there are only a few persons who have given much attention 

 to them. Among these may be mentioned Prof. Joseph 

 Leidy, Prof. D. S. Kellicott, and Dr. A. C. Stokes, each of 

 whom has done much to awaken interest in these small forms 

 and to bring them into notice. 



In order to get a good idea of the structure of the Protozoa 

 we may now consider some typical forms of the various 

 groups. 



The lowest of the Protozoa, belonging to the class Sarcodina, 

 are the subclass Rhizopoda, or root-footed animals, so called 

 because they send out a number of root-like processes of pro- 

 toplasm, known as pseudopodia, by means of which they 

 move from place to place. Among the forms included under 

 this head are Amoeba, Difflugia, Arcella, and Euglypha. 

 Amoeba consists of a small portion of protoplasm differen- 

 tiated into a granular endoplasm and a clear transparent 

 contractile ectoplasm, and having a nucleus and contractile 

 vacuole. As already indicated, the animal moves along by 

 thrusting out processes of protoplasm in the direction of loco- 

 motion. As these pseudopodia are thrust out at one part of 

 the body they are drawn in at another part. Because of 

 this peculiar movement the Ameeba has no constant form, its 

 shape changing continually. Aside from locomotion the 



