72 ZOOLOGY FOR MEDICAL STUDENTS chap. 



dividing into two gamete nuclei, and that the whole individual (gametocyte) 

 gave rise to eight gametes. In Paramecium as we now know it however 

 three of the four nuclei have ceased to function and the division into 

 distinct gametes has also disappeared from the life-history. 



Of the two functional gamete nuclei in each individual one passes 

 across into the other individual — the pellicle disappearing temporarily 

 over part of the surfaces in contact to allow of this interchange taking 

 place. The two nuclei now in each individual undergo fusion to form a 

 zygote-nucleus, the gap in the pellicle is repaired and the two individuals 

 — usually spoken of as exconjugants — separate. During the processes 

 so far described the macronucleus has remained without change but it 

 now begins to degenerate, it gradually breaks up into fragments which 

 are digested by the cytoplasm, its role having come to an end with the 

 process of syngamy. 



Each exconjugant starts life provided with a single nucleus the 

 zygote-nucleus which has arisen in the way described. In some allies of 

 Paramecium the zygote-nucleus divides into two, one of which becomes the 

 definitive rhicronucleus while the other rapidly increases in size to become 

 the macronucleus. In Paramecium itself the process while essentially 

 similar is obscured by repeated nuclear divisions followed by the degenera- 

 tion of certain nuclei and is consequently unsuited for description in 

 detail in an elementary book. 



The group Ciliata contains a vast assemblage of different kinds of 

 Protozoa which are classified into four sub-groups the scientific names 

 of which are indicative of characteristic differences in regard to the cilia. 



I. HoLOTRiCHA. This section is characterized by the body being 

 covered with a fairly uniform coating of short cilia. It is exemplified 

 by the genus Paramecium. 



II. Heterotricha. Here also there are cilia distributed over most of 

 the body surface but in this case the cilia round the edge of the peristome 

 are greatly enlarged and concentrated into groups the cilia in each of 

 which are fused together to form a ciliary plate. A good example of 

 this section is the trumpet-shaped Stentor common in fresh water 

 (Fig. 29, A). Interesting peculiarities of Stentor in detail are the presence 

 of pseudopodia at its narrow end (ps) by which it attaches itself to the 

 surface of water-plants or stones, the curious beaded (" moniliform ") 

 macronucleus (N) and numerous small micronuclei («), and the power 

 of contracting itself with great rapidity into a rounded form. This last 

 peculiarity is associated with the fact that special strands of ectoplasm 

 have become highly specialized for the function of contraction (myonemes). 



