96 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. 
threads (‘“trichocysts”). These, though parts of a cell, 
suggest the thread cells of Ccelentera, and are probably 
of the nature of weapons. The cortical layer is contractile, 
and is distinctly fibrillated. In the substance of the cell 
lie two nuclei, the smaller “micronucleus” lying by the 
side of the larger ‘‘ macronucleus.” Food vacuoles occur 
asin the Ameba. There are two contractile vacuoles, from 
which fine canals radiate into the surrounding protoplasm ; 
these discharge into the vacuole, which then bursts to the 
exterior. 
Life history.—Growth is followed by obliquely transverse 
division into two (Fig. 39, @v.). One half includes the 
“mouth,” the other has to make one. As well as this 
simple fission, a process of transient conjugation also 
occurs. -Two individuals approach one another closely, 
the two nuclei of each break up, an exchange of pieces 
of the micronucleus takes place; the two then separate, 
each to reconstruct its two nuclei (Fig. 40). This process 
is necessary for the continued health of the species. 
The details of the conjugating process have been worked out with 
great care by Maupas and others. They differ slightly in different 
species; what occurs in P. aure/éa is summarised diagrammatically 
in Fig. 41, 
The micronuclear elements are represented by two minute bodies. 
As conjugation begins, these separate themselves from the macronucleus. 
The macronucleus degenerates, and each micronucleus increases in 
size (A). Each divides into two (B); another division raises their 
number to eight (C) ; seven of these seem to be absorbed and disappear, 
the remaining eighth divides again into what may be called the male 
and female elements (D) ; for mutual fertilisation now occurs(E). After 
this exchange has been accomplished, the Infusorians separate, and 
nuclear reconstruction begins. The fertilised micronucleus divides into 
two (F), and each half divides again (G), so that there are four in each 
cell. Two of these form the macronuclei of the two daughter-cells 
into which the Infusorian proceeds to divide (H); the other two form 
the micronuclei, but before another division occurs each has again 
divided. Thus each daughter-cell contains a macronucleus and two 
micronuclei. 
Fifth Type of Protozoa—VorTicELLA 
Vorticella, or the bell-animalcule, is a type of those 
ciliated Infusorians in which the cilia are restricted to a 
region round the mouth (Peritricha). 
