The Ciliates 



(Class Ciliata) 



The ciliates are mostly free-swimming forms that 

 row themselves about by the beating of many cilia, 

 so named for their resemblance to eyelashes. Abun- 

 dant in all fresh and marine waters, ciliates flourish 

 best where there is much decaying organic material, 

 for most of them are bacteria feeders. Any water 

 dipped up from the weed-grown edge of a stagnant 

 pond, especially if it contains organic debris or frag- 

 ments of vegetation, will on microscopic examina- 

 tion reveal a miniature community in which ciliates 

 play many of the leading roles. Of all the protozoans 

 they will be the most conspicuous, move the most 

 rapidly (almost one-tenth of an inch per second, in 

 some of the fastest species), and occupy the greatest 

 variety of niches. They may be difficult to appraise 

 at a glance, for they cross the field at all angles in a 

 fast, powerful glide that can usually be slowed only 

 by using anesthetics or such a barrier as cotton fibers. 



The familiar Paramecium is often best seen when 

 anchored to a bit of debris and quietly feeding on 

 bacteria. This slipper-shaped animal has a conspicu- 

 ous groove at one side of the body, and this is lined 

 with cilia whose beating wafts bacteria and minute 

 particles of organic material through the mouth open- 

 ing into a funnel-shaped gullet. There special tracts 

 of cilia compact the bacteria into a food ball, which 

 is passed on, surrounded by a minute droplet of wa- 

 ter, as a food vacuole. Successive vacuoles are 

 launched into the lluid interior protoplasm and circu- 

 late in a regular path. One experimenter kept close 

 watch on some individuals of Tetrahymena, a small 

 relative of Paramecium, and estimated that each 

 food ball was an accumulation of about a thousand 

 bacteria and that a vacuole was sent out on its course 

 once every six minutes. The spent vacuoles were 

 eliminated some four hours later. A really large cil- 

 iate, Stentor, when placed in a rich suspension of 

 euglenas, was observed to down these flagellated 

 organisms at the remarkable rate of about a hundred 

 per minute. 



But not all ciliates feed by ciliary currents. Some 

 are predatory, actively seeking out their prey and 

 attacking it with a ferocity that matches anything 

 seen in higher animals. Didiniiim feeds mostly on 

 paramecia, devouring them whole, as many as eight 

 in one day. It has no difficulty in opening its mouth 

 wide enough to swallow any individual not too much 

 larger than itself, but if greatly outclassed in size the 

 didinium may have to struggle longer, and the vic- 

 timized Paramecium continues to swim about ac- 

 tively with the attacker grimly hanging on. In 

 clearer waters, where oxygen content is high, bacteria 

 feeders are few and carnivorous ciliates attack small 

 herbivorous ciliates that feed on green or blue-green 



algae or on diatoms. Such carnivores may in turn be 

 fed on by larger ciliated carnivores, and when the 

 carnage is all over, fragments of dead plant and ani- 

 mal tissue will be cleaned up by scavenging ciliates. 

 In this highly competitive microworld others have 

 turned to exploiting both the external surfaces and 

 internal cavities of invertebrates and vertebrates. 

 Ciliates may themselves harbor smaller parasites or 

 commensals, or they may live in a mutually beneficial 

 relationship with alga-like flagellates that carry on 

 photosynthesis within the ciliate body. 



The firm shape of most ciliates is maintained by a 

 stiff but flexible outer covering, the pellicle, and by 

 the outer clear gelatinous layer of protoplasm that 

 lies beneath the pellicle. Within the firm outer layer 

 is a more fluid, granular protoplasm in which the nu- 

 cleus floats, the food vacuoles circulate, and the con- 

 tractile vacuoles work away at pumping out the 

 excess water that accumulates more especially in the 

 particle feeders. The cilia protrude through holes in 

 the pellicle, which is handsomely marked with longi- 

 tudinal or diagonal lines of ciliary attachment. At 

 their bases, in the outer protoplasm, the cflia connect 

 with the fibrils of the neuromotor system that coordi- 

 nates ciliary movements. Much of the time the cilia 

 beat so fast that all we can see is a flickering at the 

 edge of the body. They move like the flexible arms 

 of a swimmer doing the crawl, reaching forward in 

 the relaxed part of the stroke and then striking back- 

 ward through the water in a forceful sweep — not 

 straight backward, but obliquely so that the animal 

 rotates and describes a spiral path as it continues on 

 a straight course. Aside from the ciliation of the 

 body, which serves both locomotion and feeding, 

 most members of this class share a unique nuclear 

 situation that distinguishes them from other proto- 

 zoans. The functions of the nucleus are divided be- 

 tween two separate bodies: a large nucleus con- 

 cerned with the chemical processes of feeding and 

 growth, and a small nucleus concerned with repro- 

 duction. 



Asexual reproduction occurs, as in other groups, 

 by a division of the protoplasm and each of the two 

 nuclei between two daughter cells. But sexual repro- 

 duction usually takes place by a special kind of con- 

 jugation. The individuals become sticky and pair off, 

 each couple adhering together by apposing their 

 mouths and forming a protoplasmic bridge. This 

 lasts for several hours, during which the nuclei un- 

 dergo complex changes and a portion of each small 

 nucleus migrates to the opposite member of the un- 

 ion. Thus the essential part of sexual reproduction 

 is accomplished — the recombining of hereditary ma- 

 terials so as to produce offspring with new hereditary 

 possibilities. In Paramecium, where it has been most 

 studied, the two conjugants do not appear to our eyes 

 to be visibly differentiated. But they are physiologi- 



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