96 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



food particles towards the mouth. Among the cilia there 

 are small cavities in the cortex, in which lie fine protrusible 

 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 

 as in the Amoeba. 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 liistory. Growth is followed by obliquely transverse division 

 into two (Fig. 39, D.L.). 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 micro- 

 nucleus takes place ; the two then separate, each to reconstruct its two 

 nuclei (Fig. 40). This process is necessary for the continued health ef 

 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. aurelia is summarised diagrammatical ly 

 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 VORTTCELLA 



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). 



