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place by longitudinal fission — a bell divides into similar 

 halves, one of these acquires a basal circlet of cilia and 

 goes free, ultimately becoming fixed. Or the division may 

 be unequal, and one, or as many as eight, microzooids may be 

 set free. These swim away by means of the posterior girdle 

 of cilia, and each may conjugate with an individual of normal 

 size. In this case a small active cell (like a spermatozoon) 

 fuses intimately with a larger passive cell, which may be 

 compared to an ovum. The details of the process of 

 fertilisation are analogous to those described in Paramacium. 

 It is said that in some cases an encysted Vorticella breaks 

 up into a number of minute spores, but this is doubtful. 



Sixth Type — VoLVOX. 



Volvox is a type of flagellate Infusorians, especially of 

 those with flagella of equal size. 



Volvox is found, not very commonly, in fresh water pools, 

 and is usually classed by botanists as a green Alga. It consists 

 of numerous biflagellate individuals, connected by fine pro- 

 toplasmic bridges, and embedded in a gelatinous matrix, 

 from which their flagella project, the whole forming a hollow, 

 spherical, actively motile colony. In V. globator the average 

 number of individuals is about 10,000 ; in V. aureus or 

 minor, 500-1000. The individual cells are stellate or 

 amoeboid in V. globator, more spherical in V. aureus ; each 

 contains a nucleus and a contractile vacuole. At the 

 anterior hyaline end, where the flagella are inserted, there 

 is a pigment spot ; the rest of the cell is green, owing to the 

 presence of chlorophyll corpuscles. In consequence of the 

 presence of these, Volvox is holophytic, i.e., it feeds as a plant 

 does. 



In its method of reproduction Volvox is of much biological 

 interest and importance. As Klein, one of its best describers, 

 says, it is an epitome of the evolution of sex. Some of the 

 colonies are asexual. In these a limited number of cells 

 possess the power of dividing up to form little clusters of 

 cells, these clusters escape from the envelope of the parent 

 colony, and form new free swimming colonies. In other 

 colonies there are special reproductive cells, which may be 

 called ova and spermatozoa. 



