THE ORGANIZATION OF THE PROTOZOA 55 



hand, in a purely parasitic form such as Opalina (p. 439), in which 

 a mouth is entirely absent, only locomotor cilia are present. 



The chief modifications of cilia, apart from variations in size and 

 function, are the result of a tendency to adhere or fuse together ; 

 thus arise various types of organs, of which the most common are 

 the cirri, membranettoe, and undulating membranes. Cirri are organs 

 resembling bristles, formed by fusion of a tuft of cilia, just as the 

 hairs of an ordinary camel's-hair paint-brush adhere when moistened 

 so as to form a flexible pencil. In many cases the cirri have frayed- 

 out ends, in which the component cilia are distinct from one 

 another ; and reagents often cause a cirrus to break up into 

 separate cilia. Cirri have a locomotor function, and are especially 

 characteristic of the ciliate Infusoria which are of creeping habit 

 (order Hypotricha, p. 440, infra). The cirri occur on the ventral 

 surface of the body that is to say, on the side of the body turned 

 towards the substratum on which the organism creeps, using the 

 cirri practically as legs. 



Membranellae are flapping or swinging membranes formed by 

 fusion of two or more transverse rows of cilia implanted side by 

 side, and adhering to form a flat membrane, the free edge of which 

 often has a fringed or frayed border, representing the free ends of 

 the component cilia. Membranellae occur usually in the region of 

 the peristome in spiral rows, implanted one behind the other, and 

 each membranella performs simple movements of alternate flexion and 

 expansion, comparable to those of a single cil.iu m. Both in structure, 

 origin, and movements, the membranellae must be distinguished 

 clearly from the undulating membranes presently to be described. 



Undulating membranes are sheet-like extensions of the ectoplasm, 

 which perform rippling movements, comparable to those of a sail 

 placed edgewise to the wind ; or, better still, to the undulating 

 movements performed by the dorsal fin of a sea-horse (Hippocampus) 

 or a pipe-fish (Syngnathus) when swimming. The undulating mem- 

 branes of Ciliata consist simply of a single row of cilia fused together. 

 Such membranes are found commonly in the oesophagus of In- 

 fusoria ; in the vestibule of Vorticellids there are two membranes 

 of this kind. In some genera, such as Pleuronema (Fig. 27), they 

 represent the principal food-capturing organ, and reach a great 

 development. Pleuronema swims about by means of its cilia, and 

 comes to rest sooner or later in a characteristic attitude, with the 

 cilia projecting stiffly from the body ; the large undulating membrane 

 is then protruded from the mouth, and serves by its movements to 

 waft food-particles down the oesophagus. 



Undulating membranes are also of common occurrence in the 

 Flagellata, where they are of a different type from those of Ciliata. 

 The undulating membrane in this class is always found in connec- 



