52 INVERTEBRATE ZOOLOGY 



remaining there. As a consequence, the macroconjugant now 

 has a fusion nucleus and the microconjugant which now has no 

 nucleus drops off and disintegrates. The nucleus of the remain- 

 ing individual divides into eight parts of which one becomes a 

 micronucleus and the other seven macronuclei. Through a 

 series of divisions of the micronucleus and fission of the cyto- 

 plasm, seven individuals ultimately result from the conjugation. 



Endomixis, comparable to parthenogenesis of the Metazoa, has 

 been discussed earlier in this chapter. 



Arrangement of Cilia. — On the basis of arrangement of the 

 cilia, five orders of ciliates are recognizable. The cilia on the 

 body surface are primitively arranged in meridional bands 

 radiating from a terminal mouth (as in Prorodon) but in many 

 forms the mouth has shifted its position, and with it the arrange- 

 ment of body ciliation becomes more complicated. The cilia 



Fig. 32. — Diagram showing wave motion of cilia. {After Vcrworn). 



which are not fused in bands or tufts are usually in meridional or 

 spiral arrangement and occur either in furrowlike depressions or 

 each cilium within the center of a small depression. In Para- 

 mecium, these depressions are hexagonal or rhombic. At the 

 angles of these polygons and in the middle of some of the sides 

 are located the points of the trichocysts. Each cilium is com- 

 posed of a firm axial filament covered by a sheath of more fluid 

 contractile substance. The filament takes its origin from a basal 

 granule and in many instances minute fibrils extend inward from 

 the basal granules to the wall of the macronucleus. Various 

 modifications of cilia occur. Non-motile tactile bristles have the 

 same general structure as the typical motile cilium. In the 

 pharynx of many ciliates, all the cilia of a row become fused to 

 form an undulating membrane or several adjoining rows may 

 fuse to form a membranelle. In the Hypotricha, tufts of cilia 

 are fused to form brushlike structures called cirri. 



Movements of cilia are coordinated by a system of fibrils which 

 comprise a neuromotor system. These fibrils have been most 

 highly specialized and most thoroughly studied in some of the 

 Hypotricha — Euplotes, for example (Fig. 34). Cilia which have 

 not been modified to form membranelles or cirri move in rhythmic 

 waves (Fig. 32) along the rows of unmodified cilia. 



