432 ZOOLOGY. 



vacuole. Volvox and some of its allies show animal 

 characters in the possession of a contractile vacuole and an 

 eye- spot in each cell, and in the absence of a cellulose 

 envelope. In Yolvox reproductive cells are differentiated, 

 while the other cells cannot reproduce the compound 

 organism, and thus this Flagellate forms a transition 

 towards the Metazoa. 



CLASS : INFUSORIA. 



Order Ciliata. Complex Protozoa of definite shape with 

 cilia as organs of locomotion, and a cytostome; usually 

 with undulating membranes or large rigid cilia called 

 cirrhi ; with two kinds of nuclei, a meganucleus and one 

 or more micronuclei, the latter alone taking part in conju- 

 gation (karyogamy), and giving rise after conjugation to 

 all the new nuclei. 



The above definition applies to the Ciliata proper of 

 which Paramecium is an example, but there is another 

 group called Suctoria, which are allied to the Ciliata, and 

 united with them in the Class Infusoria. The Suctoria 

 are ciliated in the young state only ; they have no mouth, 

 but absorb food through one or more tentacles which are 

 perforated at the apex. The Ciliata show the highest 

 differentiation in the cell-body which is found among the 

 Protozoa, the ectoplasm being differentiated into muscular 

 fibrils or myonemes, and often containing trichocysts 

 resembling the nematocysts of Coelenterata. They are to 

 be regarded as organisms evolved from simple cells in which 

 great complexity has been attained in the cell itself without 

 union of cells into a compound organism as in Metazoa. 

 At the same time they are not homologous with simple 

 uninucleate cells, but are multinucleate with the nuclei 

 differentiated from one another like the parts of the proto- 

 plasm. Only one micronucleus gives rise to the conjugating 

 nuclei, and at the moment of karyogamy the cell is herma- 

 phrodite, equivalent to two gametes, and two zygotes are 

 formed. Hence the two conjugating individuals are not 

 gametes, but conjugants. In Vorticella and its allies 

 one of the conjugants is small and does not produce a 

 zygote ; this has been interpreted as a step towards the 



