120 



THE RISE OF ANIMAL LIFE 



Fig. 7-13. Conjugating paramecia which have been 

 killed and stained. Note the large macronuclei and 

 the smaller micronuclei. 



25 years without conjugation ever occur- 

 ring. Just how important conjugation is to 

 the success of the race can only be conjec- 

 tured. 



Research has recently shown that within 

 a single species there are so-called "mating 

 types," which are distinguished by their 

 conjugation reactions. When known mating 

 types are mixed in a culture, the animals 

 form large clumps. They seem to become 

 covered with a sticky substance which 

 causes them to adhere to one another. After 

 an hour or so they pair off, rather securely 

 attached at their oral grooves (Fig. 7-11). 

 Very shortly the micronuclei in both ani- 

 mals divide twice in rapid succession; at 

 the same time the macronucleus starts to 

 disintegrate and disappear. Three of the 

 four nuclei in each cell degenerate, leaving 

 a single one which immediately divides 

 into a lar^e immotile micronucleus and a 



small motile one. The latter then moves 

 across a kind of protoplasmic bridge be- 

 tween the two animals and fuses with the 

 immotile micronucleus of the opposite ani- 

 mal, a process resembling fertilization in 

 higher forms. The animals then separate. 

 Three successive divisions of the fused nu- 

 cleus follow, producing eight nuclei, four of 

 which grow into macronuclei, three degen- 

 erate, and one remains as the micronucleus. 

 The paramecia and their micronuclei then 

 divide twice, eventually producing four par- 

 amecia from each ex-conjugant. This is 

 followed by ordinary fission (Fig. 7-11) un- 

 til conjugation is once more induced. 



Recently several different groups of "mat- 

 ing types" have been discovered by Jen- 

 nings and his co-workers. Individuals from 

 different groups do not conjugate, nor do 

 individuals from the same type within the 

 group. They mate only with other types of 

 their same group. From these studies much 

 is learned about the mechanics of inherit- 

 ance, an important phase of genetics which 

 will be discussed in the chapter on that sub- 

 ject. 



OTHER PROTOZOA 



This brief introduction to two different 

 Protozoa gives us a basis for considering the 

 importance of the group as a whole in re- 

 spect to numbers, variety, and classification. 

 Over 30,000 Protozoa have been described 

 as distinct species, and in numbers of indi- 

 viduals they exceed all other animals. They 

 live in water of all kinds, in soil and dust, in 

 and on the bodies of plants and animals. 

 Some of them cause the most destructive 

 diseases known to man, malaria, for exam- 

 ple. Most Protozoa are free-swimming, al- 

 though some are sessile; most live singly, 

 some form colonies. Many form a source of 

 food for aquatic animals such as fish, but 

 they are of little value to man, except a few 

 which are useful in sewage treatment. They 

 are such a large and varied group of animals 

 that some biologists have considered plac- 



