62 AN INTRODUCTION TO ZOOLOGY 



demonstrated in the flagella of several unicellular animals, a 

 flagellum being a large cilium. For example, in a Protozoon 

 Euglena (see p. 85), the flagellum consists of four fibrils which 

 extend its entire length. These are twisted about one another 

 in a spiral of two and one half turns. They can be traced into 

 the animal, where they branch out into a system of rootlets 

 (Fig. 25). In the smaller cilia these fibrils have not been seen, 



FIG. 25. Structure of the flagellum of Euglena. 

 (From Bellinger in Journ. Morph.) 



but are probably present; at least, we can account for all the 

 movements made by cilia if such a condition exists as in the flagel- 

 lum of Euglena. A fusion of cilia frequently takes place, forming 

 membranelles. In Paramecium this has occurred within the 

 mouth cavity, producing the undulating membrane (Fig. 29, Mb). 

 This is attached to the dorsal wall of the mouth, and guides the 

 food particles that are swept within its reach. 



Just beneath the cilia, embedded in the cortical layer of the 

 ectoplasm, is a uniform layer of spindle-shaped structures y^\ o" 

 mm. in length, lying with their long axes perpendicular to the 

 surface (Fig. 24, A, tr., Fig. 24, B, T). These are trichocysts. 

 Their distribution is shown in surface view in Figure 24, A. The) 7 

 appear to be cavities in the ectoplasm filled with a semi-liquid 

 homogeneous substance which is very refractive. They arise 

 in the neighborhood of the nucleus, and probably from it (72). 

 A small amount of osmic or acetic acid, when added to a drop of 

 water containing Paramecia, causes in some cases the discharge 

 of the trichocysts to the exterior through very small canals. 

 This explosion is due to the pressure derived from the contrac- 

 tion of the cortical layer of the ectoplasm. After the explosion, 



