98 PHYLUM PROTOZOA THE SIMPLEST ANIMALS. 



some Amoebae, 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 

 debris 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 witn 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 amoeboid in V. globator, more spherical in V. 

 aureus ; each contains a nucleus and a contractile vacuole. 



