162 ^"^ ANIMAL KINGDOM 



at the base of an oral depression, and is usually kept to the inside of 

 the spiral path. Food, which inchides microscopic particles such as 

 bacteria, yeast and algae, is swept into the gullet by ciliary action and 

 is cUgested in food vacuoles. The cytoplasm circulates slowly in the 

 body, so that each vacuole moves in a circle. The digestive processes 

 are like those in the ameba, and the indigestible remnants are ejected 

 through the anus, an organelle posterior to the mouth. Many of the 

 experimental strains of this genus have been cultured for years on 

 Aerobacter aerogenes, a bacterium cultured easily on boiled hay or al- 

 falfa. 



The movements of paramecia indicate highly coordinated activity. 

 When a paramecium strikes a solid object, the waves of ciliary beating 

 reverse and the animal backs up a short distance. Then it turns slightly 

 and goes ahead again. The rapidity with which the animal changes 

 direction is astonishing. Paramecia have no visible photoreceptors, but 

 they do move toward or away from a light source under certain cir- 

 cumstances. 



In addition to a network of neurofibrils (Fig. 8.3) beneath the cilia, 

 paramecia have a layer of trichocysts (Figs. 8.12 and 8.3), spindle-shaped 

 structures located between the basal bodies, that can be discharged to 

 produce long threads projecting from the body surface. All paramecia 

 and many other protozoa have them in abundance. They may be used 

 for anchorage, for the capture of prey, or for the formation of protective 

 cyst walls. 



Mating Types. Paramecia may have more than two sexes, a con- 

 dition found only in the ciliates. All the sexes look alike, but an indi- 

 vidual of a given sex will mate only with an individual of some other 

 sex. As many as eight sexes exist for a given species. Since "male" and 

 "female" are inadequate terms, the sexes of paramecia are called mating 

 types, numbered from I to VIII in the order in which they were dis- 

 covered. 



The study of paramecia is further complicated by the existence of 

 varieties, groups of mating types that interbreed among themselves but 

 which do not mate with other mating types that are morphologically 

 similar. Thus, in Paramecium bursaria (Table 2), the sixteen mating 

 types found in the United States fall into three breeding groups of 

 4, 8 and 4, respectively. From a genetic point of view these three vari- 

 eties are distinct species, since they do not interbreed. They can seldom 

 be distinguished, however, except by breeding experiments, and for 

 convenience the various morphologically similar varieties are given but 

 one species name. Most of the species of paramecia and of many other 

 ciliates are now known to be composed of several varieties. 



The Killer Trait. T. M. Sonneborn, the protozoologist at 

 Indiana University who discovered mating types, has found that the 

 inheritance of sex is determined partly by the nucleus and partly by 

 the cytoplasm, suggesting that in these animals the nucleus is not the 

 sole agent for transmitting inherited characters. The best known ex- 

 ample of cytoplasmic inheritance concerns the killer trait. In some 

 strains of Paramecium aurelia, certain individuals are able to produce 



