THE INFUSORIA l8l 



Other contractile elements are occasionally found : the most 

 remarkable, perhaps, is,the peculiar muscular band which surrounds 

 the peristome of Bursaria truncate I la. This highly differentiated 

 muscular organ, which functions as a sphincter, is, like the myonemes, 

 derived from the alveolar layer immediately below the pellicle. 



The ectoplasm appears throughout to be the seat of motion. Not 

 only are the contractile myonemes differentiations of this important 

 layer, but the cilia and all of their modifications are likewise derived 

 from it. The cilia themselves appear to be mere prolongations of 

 the alveolar layer. They are minute, probably of similar diameter 

 throughout, and except for regional differentiation in the vicinity of 

 the mouth, are of uniform length. As a rule, they are inserted upon 

 minute elevations or papillae on the cuticle and appear to be connected 

 by minute fibrils with the myonemes. The finer structure of the cilia 

 has not been satisfactorily made out, but the present results tend to 

 the view that they are simple, firm threads without differentiations. 

 Unlike flagella, they act in unison, and their motion is that of a paddle 

 rather than a lash, as in flagella. Jensen ('93) has figured the absolute 

 lifting power of the ciliary apparatus of Paramcetium at 0.00158 milli- 

 grammes, or nine times the weight of the animal. The cilia are grouped 

 together in various ways, forming more or less complex motile organs. 

 These are rarely seen in Holotrichida, but in the other orders they 

 may be pointed aggregates (cirri), plate-like vibratile organs (mem- 

 branelles), or broad, undulating membranes. All of these modifica- 

 tions are found in the Hypotrichida, where the motile apparatus is 

 especially characteristic. The arched dorsal surface is without cilia, 

 but occasionally holds a varying number of bristles which have, 

 possibly, a sensory function. On the flat, ventral side of the most 

 primitive forms of this order, the cilia are very generally distributed 

 (Peritromus, Fig. 113, B\ but in the more differentiated forms they 

 are reduced in number, and modified into cirri, membranes, and mem- 

 branelles. In many forms they may be entirely absent, the only motile 

 apparatus being the membranelles on the ventral side about the mouth. 

 These form the adoral zone, which stretches from the mouth forward 

 on the left side of the peristome, and as far as the dorsal anterior region. 

 In some cases a row of cilia stretches along the floor of the peristome 

 parallel with the membranelles, a single cilium opposite each mem- 

 branelle. The right border of the peristome (Fig. 98) is continued 

 into a vibratile membrane, and close to the left of this and running 

 parallel with it is another row of cilia (praeoral cilia poc). In the 

 centre of the peristome is a second undulating membrane, the endoral 

 membrane (em\ which passes downward and into the pharynx, and 

 this, also, is sometimes accompanied by a row of cilia even into the 

 pharynx (endoral cilia eo). 



