CHAPTER VI 



THE INFUSORIA 



"Die Infusionsthiere gehoren in den Kreis der Protozoen. Innerhalb desselben bilden 

 sie eine eigene und zwar die am hochsten stehende Klasse." STEIN. 1 



As was long since clearly recognized by Stein, the Infusoria are 

 the most highly differentiated of all Protozoa and often attain a 

 .degree of complexity which is perhaps greater than in any other 

 cells. Their form varies considerably in the several divisions, but all 

 are characterized by certain structural features by which they can be 

 distinguished at a glance. All are provided with cilia which may 

 be retained throughout life (Ciliata), or may be replaced in the 

 adult phases by suctorial tentacles, cilia being present only during 

 the embryonic phases (Suctoria); they possess mouth parts which 

 are adapted for swallowing, for simple ingestion, for sucking, or 

 which may be entirely degenerate through parasitism ; and they are 

 provided with two kinds of nuclei, known as macronuclei and micro- 

 nuclei. They reproduce by simple division and by budding, or rarely 

 by spore-formation. 



I. THE CILIATA 



Among the Ciliata the arrangement of the cilia upon the body 

 affords a character which was first used by Stein ('59), and is still 

 retained as a means of distinguishing the subdivisions of this group. 

 In the first and probably the most primitive type, HolotricJiida, the 

 cilia are arranged uniformly over the entire body of the animal and 

 show no regional differentiations (Fig. 91, A). In the second type, 

 Heterotrichida, the cilia are uniform over the main portion of the 

 body, while a specialized set fused into a curved series of firm vibra- 

 tory plates, or membranelles, are found in an adoral zone about the 

 mouth (B). In the third type, Hypotrichida, the body is flattened 

 dorso-ventrally and the dorsal side is entirely free from cilia, while on 

 the ventral side the cilia are frequently fused together into stiff seta- 

 like organs, the cirri, and as in the Heterotrichida, they may form 

 a curved line of membranelles around the mouth (C). Finally, 

 in the Peritrichida, the highest type of this class, the cilia are reduced 

 to one or two bands or girdles in addition to the adoral zone (D). 



Although, with the exception of the motile organs, no single item 

 of structure is found here which is not occasionally met with in other 



1 ('59), P- 54- 

 171 



