334 HANDBOOK OF PROTOZOOLOGY 



of small plant and animal organisms which ordinarily abound in 

 the water. Thus their nutrition is holozoic. The ciliates vary in 

 size from less than 10 microns up to 2 mm. in large forms (as in 

 an extended Spirostomum). The cytoplasm is distinctly dif- 

 ferentiated into the ectoplasm and the endoplasm. The ecto- 

 plasm gives rise to cilia and trichocysts and is covered by a 

 pellicle. The endoplasm contains nuclei, food vacuoles, con- 

 tractile vacuoles, pigment granules, crystals, etc. In the ma- 

 jority of the ciliates, the anterior and posterior extremities are 

 permanent and distinct. In all cytostome-possessing forms, the 

 oral and aboral surfaces are distinguishable, while in numerous 

 creeping forms the dorsal and ventral sides are differentiated. 



The body is covered by a very thin yet definite membrane, 

 the pellicle, which is ordinarily uniformly thin and covers the 

 entire body surface so closely that it is not recognizable in life. 

 In some forms, such as Coleps, it develops into numerous plate- 

 lets and in others, such as Trichodina, into hook-like processes. 

 The outer half of the ectoplasm may show alveolar structure 

 which, in section, exhibits radiating and parallel lines. In this 

 portion the myonemes are lodged. The deeper layer of the 

 ectoplasm is structureless and free from granules. In the ecto- 

 plasm are embedded the basal granules of cilia, which are ar- 

 ranged in longitudinal, oblique, or spiral rows. The cilia may 

 fuse to form cirri, membranellae, and undulating membranes 

 which are invariably present in certain groups. In Euciliata 

 contractile vacuoles with one to several radiating canals are 

 one of the prominent structures. The endoplasm is more fluid 

 in nature and the ground substance is finely granulated or 

 reticulated. It undergoes rotation movement, called cyclosis. 



Two types of nuclei are present in all Euciliata. The massive 

 macronucleus is of various forms. In Paramecium it is oblong; 

 in Vorticella it is band-form, often curved; in Stentor it is 

 beaded. The chromatin granules fill compactly the intranuclear 

 space. The macronucleus multiplies by amitosis as was stated 

 before. The micronucleus is ordinarily so minute that it is diffi- 

 cult to see in a living specimen. It is spherical and usually ve- 

 sicular in structure, although in some it appears to be compact, 

 and consists of a centrally located endosome, the nucleoplasm, 

 and the membrane. The number of the micronucleus present in 



