DERIVED ORGANIZATION— TAXONOMIC STRUCTURES 155 



giving the striped appearance characteristic of the ciliates. Waves 

 of contraction pass from the anterior end posteriorly, cilia of the 

 same transverse rows beating synchronously, those of the same 

 longitudinal rows metachronously. 



The periplast is variously sculptured in different species, giving 

 the appearance superficially of a different mode of origin of the 

 cilia. In some cases they appear to come from the centers of 

 minute cups or dimples as in Paramecium aurelia; in other cases 

 from longitudinal grooves or furrows between ridges of periplast 

 (Fig. 69, p. 124), and in some they appear to come from the ridges 

 themselves. 



Rhizoplasts or endoplasmic prolongations from the basal bodies 

 are comparatively rare but occur in some cases as in Didinium 

 nasutum (Fig. 98, p. 187). Coordinating fibrils apart from the 

 silver line system have been described in a few types (En plaits, 

 Diplodinium, see p. 129), and center in a specialized neuromotor 

 body, the so-called motorium (Yocom, Taylor, Sharp). 



In some cases cilia are uniform in length over the entire body 

 (Opalina); in other oases they are longer in the region of the mouth 

 or around the posterior end, but no sharp dividing point separates 

 short from long ones (Fig. 84). In some cases they are uniformly 

 long and vibrate like flagella (Actinobolus radians, Fig. 91, p. 163). 



4. Composite Motile Organs.- A well-marked characteristic of cilia 

 is the ability of two or more to fuse into motile organs of vari- 

 able complexity. Such combinations give rise to membranulae, 

 membranelles, undulating membranes and cirri, each of which, 

 although composed of fused cilia, originates or grows as an inde- 

 pendent and complete organoid. In each case also the component 

 cilia may be demonstrated by use of dilute alkalies such as potas- 

 sium or sodium hydrate. It is often difficult to distinguish lines 

 of closely set cilia from fused cilia, and loosely bound cilia are 

 sometimes present, the aggregates being spoken of as "pseudo- 

 membranes." 



Membranulae.— Membranulae are very long, delicate, finely- 

 pointed aggregates of cilia which differ from the somewhat similar 

 cirri in movement and in composition, while their basal granules, 

 in Didinium nasutum at least, are connected with the vicinity of 

 the nucleus by definite rhizoplasts (Fig. 98, p. 187). Similar mem- 

 branulae form the basal ring in Vorticellidae (Schroder, Schuberg, 

 etc.). 



Membranelles. — Membranelles are formed by the fusion of cilia 

 in the region of the mouth. In many of the Holotrichida the cilia 

 are longer just posterior to the mouth than in other regions of the 

 body, frequently forming circlets about the mouth as in Lacrymaria 

 olor or L. lagenula (Fig. 85). In the other Orders of Ciliata oral 

 cilia are fused to form membranelles. In the oral regions the body 



