94 PROTOZOA — THE SIMPLEST ANIMALS. 



Sometimes a Vorticella bell jerks itself off its stalk and 

 swims about ; in other conditions it may form a temporary 

 cyst ; normally, the cilia are very active, and the movements 

 of the stalk frequent and rapid. Multiplication may take 

 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 sper- 

 matozoon) 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 Para- 

 mecium. 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 protoplasmic 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 



