482 PROTOZOOLOGY 



Spirostomuin or Stciitor). The cytoplasm is distinctly differen- 

 tiated into the ectoplasm and the endoplasm. The ectoplasm gives 

 rise to the cilia and trichocysts and is covered by a pellicle. The 

 endoplasm contains nuclei, food vacuoles, contractile vacuoles, 

 pigment granules, crystals, etc. In the majority of 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 platelets and in 

 others, such as Trichodina, into hook-like processes. The outer 

 half of the ectoplasm may show alveolar structure which, in sec- 

 tion, exhibits radiating and parallel lines. In this portion the 

 myonemes (p. 51) are lodged. The deeper layer of the ectoplasm 

 is structureless and free from granules. In the ectoplasm are em- 

 bedded the basal granules of cilia, which are arranged in longi- 

 tudinal, oblique, or spiral rows. In recent years complex fibrillar 

 systems have been recognized in many ciliates (p. 55-59). The cilia 

 may fuse to form cirri, membranellae, and undulating membranes 

 which occur in certain groups. In many euciliates contractile 

 vacuoles with one to several collecting canals are one of the 

 prominent structures. The endoplasm is more fluid and the 

 ground substance is finely granulated or reticulated; it undergoes 

 rotation movement or cyclosis. 



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

 macronucleus is of various forms. The chromatin granules which 

 may reach 20^ in diameter (p. 34) fill compactly the intranuclear 

 space. The macronucleus multiplies by amitosis. The micronu- 

 cleus is ordinarily so minute that it is difficult to see in a living 

 specimen. It is vesicular in strusture, although in some it appears 

 to be compact, and consists of an endosome, the nucleoplasm, and 

 the membrane. The number of micronuclei present in an indi- 

 vidual varies among different species. At the time of reproduction 

 it increases in size and divides mitotically; during conjugation it 

 undergoes a meiotic division (p. 158). 



The protociliates possess from one to many hundreds of nuclei 

 of a uniformly same structure and numerous ovoid or spindle- 



