DERIVED ORGANIZATION— TAXONOMIC STRUCTURES 157 



the mouth. In every order except the Holotrichida a fringe of such 

 specialized motile organs, known as the adoral zone, lies on a margin 

 of the peristome (Fig. 88). 



Membranelles are usually made up by the fusion of two rows of 

 cilia as shown by the double row of basal bodies (Maier) and their 

 flat or curved faces make powerful sweeps in the water. According 

 to Schuberg, Gruber, Maier and others, the anchorage of these 

 organoids is quite complex. The basal granules form a double row 

 immediately below the periplast; fibrils from these, analogous to 

 rhizoplasts, form a broad triangular basal plate and are then brought 

 together to form an end thread which connects the membranelle 

 with coordinating fibers (Fig. 72, p. 130). 



While in most cases the membranelles represent the fusion of 

 comparatively few cilia in transverse rows of the peristome, making 

 them relatively narrow at the base, in other cases, notably in the 

 Tintinnidae, such fusion includes practically all of the cilia of the 

 transverse rows, making membranelles as broad as the peristome. 

 In the Vorticellidae there are two rows of membranelles, the double 

 adoral zone winding about the peristome usually in a direction 

 opposite to that of the Heterotrichida and Hypotrichida (Fig. 86.) 



Undulating Membranes. — Undulating membranes are found in all 

 orders of the ciliates and range in size from delicate aggregates no 

 broader from base to tip than ordinary cilia to relatively enormous 

 balloon-like structures equal in width to more than half the diameter 

 of the body and in some cases, as Lembadion conchoides, almost equal 

 to length of the body (Fig. 87). In the simplest cases these mem- 

 branes are composed of a single row of longitudinally placed cilia, the 

 basal bodies of which form a single basal strand. Since cilia of the 

 longitudinal rows beat metachronously the result of their contrac- 

 tion when fused in these undulating membranes is a series of waves 

 passing from the anterior to the posterior end. In more complex 

 forms undulating membranes may be composed of 3 to 10 rows of 

 cilia, fused in longitudinal rows, the length varying from a few 

 microns to great waving sheets of protoplasm almost as long as 

 the entire cell (Fig. 87). They are usually found in the peristomial 

 area inside the adoral zone and are named preoral, endoral, paroral, 

 etc., according to their positions in relation to the mouth. 



Pseudomembranes are present in numerous types. Here the 

 component cilia are not firmly united and the membrane is easily 

 disrupted. Such a membrane, which is rather easily disintegrated, 

 is characteristic of Blepharisma undulans. Chambers and Dawson 

 (1925) were able to hold down a portion of the pseudomembrane 

 with a needle whereupon the distal portion broke into fibrils which 

 later reunited after the obstruction was removed. 



Cirri. — Cirri are the most highly specialized of all the motile 

 organs of ciliates, the most characteristic forms occurring in the 



