164 



THE ANIMAL KINGDOM 



mitted to the offspring so long as reproduction of the particles keeps 

 pace with tiiat ol the paramecia. Thus, a trait may be acquired and 

 transmitted to the progeny. The ability to acquire the trait is absent in 

 some strains, and Sonneborn has shown that this depends upon the 

 presence in tlie nucleus ol appropriate genes, indicating that this kind 

 ol cytoplasmic inheritance is ultimately controlled by the nucleus. 



Tetrahymena. Related to Faraniecimn is a genus of smaller ciliates, 

 Tetra/tyinena. These are similar in many respects, especially in the 

 complexity of varieties and mating types that occurs. One species, Tetra- 

 JiynieJKi pyriforrnis, is of special interest because it can be cultured on a 

 li(juid medium in which the exact amounts and kinds of all the dis- 

 solved chemicals are known. By varying the chemical nature of some 

 of the ingredients, scientists are discovering not only what materials are 

 essential for growth and maintenance, but to what extent they may be 

 converted into other materials, and something of the steps involved in 

 these transformations. This may seem to be extravagant detail in the 

 study of a mere protozoan, but it is now clear that the metabolic path- 

 ways in all organisms are essentially alike, and the study of this animal, 

 in which information can be obtained rapidly and with relative ease, 

 is shedding light on similar problems for all organisms, including man. 



Other Ciliates. Paramecium and Tetrahymena belong to the order 

 Holotricha, including ciliates completely or partially covered with 

 simple cilia. In many the cilia of the gullet or near the mouth are fused 

 together to form small, flaplike membranelles. In the order Spirotricha 

 the membranelles are large and are arranged in a clockwise spiral lead- 

 ing to the mouth. Examples of this order are the common hypotrichs 

 (Fig. 8.13) which lack cilia on the upper surface. On the lower surface, 

 in addition to the adoral membranelles, are patches of cilia fused into 



"Ventral 

 cirrus 



Hypotrich Pcritrich 



Figure 8.13. Other ciliates. The hypotrichs (order Heterotricha) run rapidly on the 

 ventral cirri, formed by the fusion of cilia. The peritrichs (order Peritricha) are mostly 

 sessile and feed by sweeping food toward the mouth. 



