GENERAL SKETCH 



45 



characteristic of various epithelial tissues in Metazoa, they are found 

 only in a single specialized group of the Protozoa, the Infusoria. In 

 form, they are similar to flagella, hut as a rule they are shorter, never 

 pointed, and more numerous, many of them acting in unison, with 

 a quick regular motion like a set of oars. In some groups, the cell 

 is completely clothed with these motile elements (Holotrichida), in 

 others only a portion is covered either in one or more rings about the 

 body (Peritrichida), or upon one surface only (Hypotrichida). The 

 cilia may also become variously modified by fusion with one another, 



111 



a 



Fig. 16. — Cilia and myonemes of Infusoria, [a, b, and <-, JOHNSON; c, d, /, g, BtiTSCHLI.] 

 The surface view of Stentor casruleus (c, e) shows rows of cilia inserted on the borders of canal- 

 like markings, each of which contains a myoneme (d). These are more clearly shown in the 

 optical section (/). In Holophrya discolor (g) the canals and myonemes are inserted deeper in 

 the cortical plasm, a, the membrane of Stentor casruleus under pressure. 



giving rise to motile organs of a more complex structure, such as the 

 membranes, membranelles, and cirri found in different groups of the 

 ciliated Infusoria. 



In many Protozoa the adult forms have no distinct motile organs, 

 although they may pass through embryonic stages in which such 

 structures are present. The Suctoria, for example, are, for the 

 most part, entirely devoid of cilia in the adult stages, although the 

 embryos possess them. Again, many of the Rhizopoda pass through 

 flagellated stages before assuming the amoeboid condition, and certain 

 Mastigophora pass through amoeboid swarm-spore stages. Some 

 Heliozoa and Radiolaria similarly pass through both flagellated and 

 amoeboid stages before assuming their own adult forms. 



Normal movement on the part of Protozoa provided with pseudo- 



