240 FRESH-WATER BIOLOGY 



surface to some support. Others are attached by stalks or pedicels 

 which may be rigid, flexible or, in some forms, as Vorticella, may 

 contract spirally. Special organs of defense are provided in a few 

 flagellates and many ciliates in the form of trichocysts or stinging, 

 thread-like structures. In at least one genus of flagellates, Poly- 

 krikos, the stinging threads are highly specialized, resembling 

 nematocysts of Coelenterata. As in Sarcodina, one or more con- 

 tractile vacuoles are usually present in the flagellates and ciliates, 

 their function being similar in all Protozoa. 



Nuclei are present in all Mastigophora and Infusoria but con- 

 siderable structural variation exists with respect to them in these 

 two groups. In some flagellates the nucleus consists of scattered 

 or grouped particles of chromatin without a nuclear membrane, 

 while in many of the higher Infusoria it consists of a highly differ- 

 entiated, branched structure. Infusoria differ from other Proto- 

 zoa, with a few possible exceptions, in the possession of two kinds 

 of nuclei in each cell, a macronucleus and a micronucleus, the 

 former being concerned with the vegetative functions and asexual 

 division, the latter with sexual division. The macronucleus is the 

 larger and often varies greatly from the regular spherical type; the 

 micronucleus is usually very small, spherical, and in close contact 

 with the macronucleus. In but one flagellate, Polykrikos, has this 

 differentiation into two nuclei been found. In the key which 

 follows, wherever the term nucleus is mentioned, reference is made 

 to the macronucleus. In many forms of Mastigophora and In- 

 fusoria as well as Sarcodina, the nucleus encloses a spherical body 

 which functions as a division center. During the resting stage of 

 the cell the division center resembles a nucleolus in appearance, but 

 during mitosis it elongates, forming a spindle, and indirect division 

 comparable to that in the Metazoa occurs in some of the more 

 complex forms. 



The endoplasm of many Mastigophora encloses colored corpuscles 

 or chroma tophores, green, yellow, and brown being the prevailing 

 colors. The chromatophores themselves often enclose deeply 

 staining pyrenoid bodies which probably have to do with the con- 

 struction of starch. Other inclusions as oil droplets, paramylum 

 granules, allied to starch, and pigment spots are common in those 



