58 



THE PROTOZOA 



general rule they are visible more or less clearly in the larger, but 

 not in the more minute, species. Thus, in trypanosomes, myonemes 

 can be made out in large forms as delicate lines running parallel to 

 the undulating membrane (Fig. 28), but in small species of trypano- 

 somes it may be impossible to discover them, although the nature 

 of their movements may leave no doubt as to the existence of con- 

 tractile mechanisms in the ectoplasm. In other cases, both motile 

 species possessing myonemes and non-motile species lacking them 

 may occur within the limits of a single group, as in Gregarines, 

 where the motile species show a very distinct layer of myonemes 



(Fig. 29); while the non-motile 

 forms have a much thinner ecto- 

 plasm, represented practically by 

 the cuticle alone, with no trace 

 of myonemes. In the non-motile 

 trophozoites of the Coccidia myo- 



Fio. 28. Trypanosoma pe COB, stout 

 form stained with iron-haematoxylin 

 to show myonemes. After Minchin, 

 X 2,000. 



FIG. 29. Qregarina munieri, showing 

 the layer of myonemes at the surface 

 of the body, slightly diagrammatic. 

 After Schneider. 



nemes are similarly absent. In the ciliate Infusoria the myonemes 

 run parallel to, and beneath, the rows of cilia, and in species of 

 large size and great powers of contractility, such as Stentor, the 

 myonemes are lodged in canals and show a transverse striation 

 (Fig. 186, /). 



According to Schaudinn, these motile mechanisms, both flagella 

 and myonemes, are derived from the achromatic spindle of a 

 dividing nucleus. In the development of a trypanosome from a 

 non-flagellated condition, he describes the entire kinetic apparatus 

 as arising from a nuclear spindle consisting of two polar centro- 

 somes connected by a centrodesmose (p. 103, infra), and by mantle 



