PROTOZOA. 55 



may last five or six days or even longer. This union 

 brings about important changes, the nucleus is broken 

 up, the paranuclei divide, and protoplasm may be inter- 

 changed as wejl as paranuclei. The two zoons then sep- 

 arate and a reconstruction of the parts takes place with 

 rejuvenescence of the organs followed by fission. 1 



While Paramoecium is a free-swimming single form in 

 youth and adult life, Stentor polymorphus Mull. (PI. 55, 

 figs. 1-3) is sometimes a single swimmer when young 

 and often a stationary and colonial form when full grown. 

 The little embryo of Stentor (fig. i) is nearly spherical in 

 shape (the ground form of most Protozoa). Its cilia, 

 even when within the body of the parent, are developed, 

 but it possesses neither a mouth nor an esophageal tube. 

 In time, however, these appear (fig. 2) ; the rounded 

 body becomes trumpet-shaped and is often attached to 

 some object. The Stentor then secretes a mucilaginous 

 sheath about the posterior tubular portion of its graceful 

 body (fig. 3). The upper anterior expansion of the 

 trumpet has the delicate wreath of cilia and the large 

 cilia near the mouth describe a spiral. The mouth ex- 

 tends into a spiral tube. 



The protoplasm of Stentor has become differentiated to 

 form a layer of thread-like fibrillae which extends from 

 the anterior to the posterior end of the body and is 

 extremely elastic. Another set of these fibrillae surrounds 

 the ciliated disc and helps to close this region when the 

 Stentor is contracted. This differentiated layer of elastic 

 fibrillae is probably the initial form of the muscular sys- 

 tem of the more specialized animals. The long beaded 

 nucleus is seen at the right of PI. 55, fig. 3. Unlike most 



1 According to Eigenmann (Bull. U. S. Fish Commission, XII, 

 1892), the ciliate Infusoria have two nuclei, the macronucleus and 

 the micronucleus, the former of which is probably represented by 

 the yolk nucleus of the Metazoa. This author gives a diagram 

 showing the maturation, conjugation, and segmentation of Protozoa 

 and Metazoa. 



