cally distinct, as it has been shown that pairing oc- 

 curs only when the two members are from different 

 strains. 



Only a little of the behavior of ciliates is of the 

 kind that we see in higher animals as they move di- 

 rectly toward some favorable object or situation in 

 their environment. Ciliates secure very little warning 

 of what surrounds them, beyond what current feed- 

 ers can detect as they sample the water ahead, so 

 that mostly they find the best conditions for existence 

 by a kind of trial-and-error method, as we do when 

 fumbling about in the dark. A Paramecium does not 

 react when entering a favorable situation but only 

 when it starts to leave such an area. That is, on 

 reaching a place where it can detect physical obstruc- 

 tion, too high or low a degree of acidity, or too high 

 or low a temperature, it backs up by reversing the 

 ciliary beat, and then changes its course slightly. If 

 it meets the same unfavorable stimulus it backs up 

 again and again shifts its course. We have named 

 this the "avoiding reaction,"' and it can be repeated 

 any number of times until the animal at last finds a 

 free passageway or is turned back into the more fa- 

 vorable region in which it has been moving. Unlike 

 those people who go about boasting that they know 

 what they like, ciliates seem mostly to know only 

 what they do not like. And they make their way 

 about in life by getting "trapped" in those areas 

 where they are best adapted to live. 



As a group the ciliates are the most highly special- 

 ized of the protozoans, with a variety of feeding 

 structures and of elaborate locomotory and coordi- 

 nating systems that defy brief description. The cili- 

 ation of the body ranges from an even covering over 

 the whole body, as in the holotrichs, to a few large 

 cilia on the lower surface, as in the hypotrichs. On 

 the basis of the distribution of the cilia, the class has 

 been divided into a number of orders. 



THE OPALINIDS 



The opalinids, named for their beautiful opales- 

 cent appearance, are all mouthless parasites, mostly 

 of the large intestines of amphibians. When removed 

 from the rectum of the frog or toad, they will be seen 

 swimming about by what appear to be short cilia 

 that clothe the whole body. Opalina is oval, flattened, 

 and has many similar nuclei. In the spring, at just 

 about the time that frog eggs are hatching into tad- 

 poles, Opalina produces cysts that pass out in the 

 feces. Those that happen to be swallowed by a tad- 

 pole hatch and give rise by a sexual process (not the 

 conjugation seen in other ciliates) to a new genera- 

 tion of opalescent adults that absorb food in the frog 

 intestine. In spite of their superficial appearance 

 they are often put with the flagellates, which they 

 resemble in many ways, notably in the habit of di- 

 viding lengthwise — instead of crosswise, as the cili- 



ates do. Moreover, their nuclei, which number from 

 two to many, are all alike instead of being of two 

 kinds, as in most ciliates. 



THE HOLOTRICHS 



The holotrichs typically have simple cilia, usually 

 short and of equal length, that cover the whole body 

 in lengthwise rows, as in Paramecium. Or the cilia 

 may be restricted to certain areas of the body, as in 

 the two or more ciliary girdles that encircle the bar- 

 rel-shaped Didinium, and the rows of cilia that 

 emerge between the plates of the armor that enclose 

 its near relative Coleps. To follow the group tradition 

 is to earn a living by bacteria feeding, and most have 

 a ciliated groove or depression that funnels food into 

 the always open mouth, reversing the ciliary beat if 

 what comes in is undesirable. Some are predators, 

 however, and Didinium and Coleps have at the front 

 end a mouth that can be opened wide to swallow 

 large prey. In Didinium nasutuni the mouth is at the 

 top of an extensible proboscis which is not fully pro- 

 truded as the animal swims about, barging full on 

 into anything that comes in its way. Didinia have 

 been seen to strike the glass walls of aquaria, algae, 

 euglenas, rotifers, the giant ciliates Steiitor and Spi- 

 rostomum — all without success. When, however, 

 they happen to strike Paramecium or another com- 

 mon holotrich, Colpoda. the proboscis penetrates 

 and fastens onto the victim, drawing it whole into 

 the widely spread mouth. Vorticella, a bell-shaped 

 ciliate, also gives way to the snout, as does frontonia, 

 though this last is a very large holotrich and has to 

 be eaten in installments. Mast, a leading American 

 student of feeding in protozoans, saw one frontonia 

 attacked in fifty-eight places by many didinia, but 

 until it died after forty minutes it kept closing its 

 wounds and swimming about actively, growing 

 smaller and smaller all the time. Apparently the 

 choice of food in didinia depends not on toughness 

 of exterior or on size but on particular physical or 

 chemical properties of the prey. 



When a didinium attacks its usual food, a Para- 

 mecium, the victim may shoot out a barrage of trich- 

 ocysts, long, sticky threads extruded through pores 

 in the outer covering. The undischarged trichocysts 

 lie as a layer of carrot-shaped bodies in the outer 

 protoplasm; and many physical or chemical stimuli, 

 especially irritants, will cause their discharge. They 

 may be defensive devices, but they make a didinium 

 stand back only briefly. In the end the paramecium, 

 enveloped in gelatinous threads, goes down the hatch. 

 It takes more than trichocysts to discourage an ani- A 

 mal like the didinium that was seen to devour two " 

 conjugating paramecia at one time. In a Parame- 

 cium, at least, the trichocysts seem most useful as a ^ 

 means of anchoring the animal as it quietly feeds on fl 

 bacteria. Though common in holotrichs, trichocysts 



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