76 PROTOPLASM OF PROTOZOA 



seems to be walking on the solid surface, the two membranelles moving 

 one after the other, somewhat like the cirri of the hypotrichs. The mem- 

 branelles are not contractile but very elastic; the Stromhidium may un- 

 fasten itself all at once and swim hastily away. Similar fixing organs have 

 been described for other oligotrichs (5", urceolare and S. clavellinae) . 

 Metacystis lagenula retracts by means of a large filament (a modified 

 cilium glazed with sticky protoplasm), which adheres to the interior of 

 the test, while M. recurva and Vasicola gracilis seem to be kept in their 

 tests by a specialized cilium, a veritable bristle, which adheres to the 

 inside of the test (Penard, 1922). Mesodin'nim pulex may adhere by 

 means of tentacle-like structures, and various species of Stentor attach 

 themselves to the substrate by means of cilia (Faure-Fremiet, 1910) 

 which form together with ectoplasmic extensions the so-called pseudo- 

 podia of the attaching disk of this form. The presence of slime-like or 

 mucus secretions for attachment have been described for Spirostomum 

 (Jennings, 1906), Urocentrum turbo and for Strombilidium gyrans. In 

 the latter the slime, which is secreted by an attaching organ, the scopula, 

 derived from modified cilia, congeals upon contact with the water into 

 a very resistant thread which is attached to some structure in the medium 

 (Faure-Fremiet, 1910; Penard, 1922). The Strombilidfum may then 

 swing back and forth like a pendulum, held in position by the thread 

 of slime which can be seen only because small particles of debris in the 

 culture medium adhere to it. Certain observations of Chambers and 

 Dawson (1925) on Blepharisma seem to show that the ability of cilia 

 to combine into composite organelles, such as undulating membranes, 

 membranelles, cirri, and so forth, may be dependent upon the presence 

 of a slime-like secretion which spreads over the cilia and joins them into 

 what looks like a homogeneous structure. At any rate these composite or- 

 ganelles are often seen to break down into their constituent cilia, after 

 which they may be recombined. This separation of composite motor 

 organelles into their components often takes place upon fixation. 



According to Mast ( 1 909 ) , the prey of Didin'mm is held by means of 

 the seizing organ, which in some way adheres to the surface when con- 

 tact is made. Capture and ingestion of food depend upon the adherence 

 of the seizing organ of Didinium and the strength of the ectoplasm of 

 its prey, which is usually Paramecium. There are two different explana- 

 tions of how the tentacles of the Suctoria adhere to their prey: by the 



