The large and rather heterogeneous group of organisms, 

 known as Infusoria,* may be traced back to a point of origin 

 at the top of the Monera, and the ancestral forms were 

 probably simple Monads, resembling closely the mastigopod 

 stage in the life-history of Protomyxa, If one of the doubt- 

 less numerous varieties of Protomyxa-like organisms, which 

 have existed, had its mastigopod stage emphasised, so as to 

 become the most important condition in its life, while the 

 other stages were partly suppressed or modified, it would 

 gradually become a Monad, or simple Flagellate Infusorian 

 (fig. 3), and such was probably the mode of origin of this 

 group of the Protozoa. 



Fig. 3. A Monad, one of the Flagellate Infusorians. 



The main line of the Infusoria leads upwards from 

 these simple ancestral forms to the very much higher 

 and more complex Ciliata; but there are several aberrant 

 groups, such as some of the Flagellata, the Catallacta 

 {Magosphcera), Noctiluca, and allied forms, which must have 



Fig. 4. MagosphcBra planula, Haeckel. Optical section of the colonial stage in the 



life-history. 



diverged from the axis far back, and have become evolved 

 in different directions. Magosphcera (fig. 4) exhibits affini- 



* For figures of this group, the student should consult Saville Kent's 

 Mamial of the Infusoria. 



