224 ] The Classification of Lower Organisms 



are essentially uniform and uniformly distributed on the surface. Other examples 

 are without separate cilia upon part or all of the surface, but bear a variety of struc- 

 tures which consist of coalescent cilia. Membranelles are triangular appendages con- 

 sisting of brief rows of ciHa; undulating membranes represent long rows; cirri represent 

 tufts. The organisms of class Tentaculifera bear cilia only in the juvenile condition. 

 At maturity they bear extensible tubular structures called tentacles, by means of 

 which they capture free-swimming ciUates and absorb their contents. 



The basal granules of the cilia are linked together by a system of fibrils; the cilia 

 and fibrils make up the neuromotor apparatus. This term was coined by Sharp, in his 

 study of Diplodinium (1914). The neuromotor fibrils form a highly elaborate net- 

 work, not connected with the nucleus, as in flagellates, but to a central structure, 

 apparently regulative, called the motorium. The motorium of Diplodinium is a mas- 

 sive body near the anterior end; that of the tintinnids is fairly large in proportion to 

 the cells (Campbell, 1926, 1927); that of Paramaecium, presumably a comparatively 

 primitive organism, is a minute body lying near the dorsal side of the cytopharynx 

 (Lund, 1933). 



Imbedded in the pellicle, in addition to the neuromotor fibrils, there are certain 

 minute ellipsoid bodies called trichocysts. These, when the cell is irritated, discharge 

 their contents in the form of elongate rods or threads. Their mechanism and effect 

 are not understood. 



In most Ciliophora, each cell has a mouth and gullet; or better, since these struc- 

 tures are not homologous with those of animals, a cytostome and cytopharynx. The 

 cytopharynx is a more or less funnel-shaped impression in the cell. It is bounded 

 laterally by ciliate pellicle; its outer opening is the cytostome; it is closed at the inner 

 end by a layer of cell membrane directly over fluid cytoplasm. Prey, chiefly bacteria 

 and small algae, encountered by the organism as it swims, is swept into the cyto- 

 pharynx by the action of the cilia. When a certain mass of prey has accumulated, 

 the cell membrane at the inner end of the cytopharynx becomes impressed and under- 

 goes constriction, enclosing the prey in a food vacuole. The material in the food 

 vacuole undergoes digestion; while this is taking place, movement of the cytoplasm 

 carries the vacuole along a definite circuitous course through the interior of the cell. 

 After some time, the vacuole arrives at a certain point on the pellicle, the anus or 

 cytoproct, where it discharges its contents and disappears by bursting through the 

 pellicle. 



In freshwater species, each cell contains one or more contractile vacuoles which 

 appear at definite points and disappear periodically by discharge of their contents 

 to the exterior. Associated with the proper contractile vacuoles, there may be systems 

 of "canals" which are in fact additional contractile vacuoles. These structures have 

 been much studied; there are notable accounts by Day (1930) and Mac Lennan 

 (1933). When a vacuole has disappeared by discharge, it reappears as one or more 

 minute vacuoles in the same area: minute bodies of gelled protoplasm turn into sol, 

 and then become lifeless liquid. The discharge of a "canal" into the proper con- 

 tractile vacuole occurs by dissolution of the bounding membranes of gelled proto- 

 plasm where the two are in contact, followed by contraction of the membrane of the 

 canal. The proper contractile vacuole discharges by essentially the same mechan- 

 ism. Its membrane meets and becomes fused with the bounding membrane of the 

 cell, generally at the end of a brief channel through the pellicle; the combined mem- 

 brane breaks, and the membrane of the vacuole contracts. 



