38 MANUAL OF ZOOLOGY SECT. 



directed upwards. In most cases the body is equal-sided 

 or bilaterally symmetrical, i.e., is capable of being divided 

 into two equal lateral portions along the median vertical 

 plane ; but sometimes it is unsymmetrical, one side differ- 

 ing more or less from the other. In most the body is, as 

 in Euglena, naked ; but some have a chitinoid shell or 

 lorica, while others have a firm cell-wall of cellulose which 

 may present an elaborate pattern of stripes, dots, &c., and 

 may be produced into long processes. Most of the Mastigo- 

 phora are, like Euglena, free-swimming ; but some are 

 permanently attached by means of a slender stalk (Fig. 13, 

 10, /j, 14-, Fig. 14. /, j). 



. The number and arrangement of the flagella vary greatly. 

 The number may be one, as in Euglena, or two, three, or four. 

 In forms with two flagella these are both attached at or near 

 the anterior end, and often take on different functions, one 

 of them, directed forwards, being alone used in locomotion, 

 while the other is trailed behind when the animal is swim- 

 ming freely, or is used to anchor it to various solid bodies. 

 In one large group of Mastigophora, the Choanoflagellata 

 (Fig. 14), there is, surrounding the base of the flagellum, a 

 remarkable vase-like prolongation of the protoplasm, ex- 

 ceedingly delicate and transparent, called the collar. This 

 is contractile, and, though its precise functions are not yet 

 certainly known, there is evidence to show that its move- 

 ments cause a flow of water, with minute particles in sus- 

 pension, up the outside of the collar and down the inside, 

 the solid particles being then ingested in the soft protoplasm 

 between the base of the flagellum and that of the collar. 

 Both collar and flagellum may be withdrawn, and the animal 

 pass into an Amoeba-like or ama'boid form. In another 

 group the Dinoflagellata (Fig. 15) there are two flagella, 

 one springing from a longitudinal groove extending along 



