98 PHYLUM PROTOZOA—THE SIMPLEST ANIMALS. 
some Amcebee, Gregarines, spermatozoa, etc., and of a much 
coarser type in striped muscle fibres. It seems to be some 
structural adaptation to contractility.) The bell has a 
thickened margin, and within this lies a disc-like lid; in 
a depression on the left side, between the margin and the 
disc, there is an opening, the mouth, which leads by a 
distinct passage into the cell. On the side of this passage 
there is a weak spot, the potential anus, by which useless 
débris is passed out. The cilia are arranged so as to waft 
food particles into the mouth and down the passage. 
There is a large and horseshoe-shaped macronucleus, and 
a small micronucleus. Food vacuoles and contractile 
vacuoles are present as usual. 
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 move- 
ments 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 spermatozoon) fuses intimately with a larger passive 
cell, which may be compared to an ovum. 
Sixth Type of Protozoa—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 amceboid in VM. globator, more spherical in V. 
aureus; each contains a nucleus and a contractile vacuole. 
