684 PROTOZOOLOGY 



In the majority of ciliates, the anterior and posterior extremities are 

 permanent and distinct; in all eytostome-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 section, 

 exhibits radiating and parallel lines. In this portion the myonemes 

 (p. 61) are lodged. The deeper layer of the ectoplasm is structureless 

 and free from granules. In the ectoplasm are embedded the kineto- 

 somes of cilia, which are arranged in longitudinal, oblique, or 

 spiral rows. In recent years complex fibrillar systems have been 

 recognized in many ciliates (p. 63-70). The cilia may fuse to form 

 cirri, membranellae, and undulating membranes (p. 59) 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 granu- 

 lated 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/x in diameter (p. 42) fill compactly the intranuclear 

 space. The macronucleus multiplies by amitosis. The micronucleus 

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

 It is vesicular in structure, although in some it appears to be com- 

 pact, and consists of an endosome, the chromatin, 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 under- 

 goes a characteristic meiotic division (p. 206). 



The protociliates possess from two to many nuclei of a uniformly 

 same structure and numerous ovoid or spindle-shaped bodies, endo- 

 spherules, the nature of which is open to speculation. Some authors 

 think that they are nuclei (micronuclei (after Hickson, 1903) or 

 macronuclei (after Konsuloff, 1922, 1930)); others consider them as 

 reserve food materials (Patten). Metcalf (1909) considers that each 

 nucleus possesses both metabolic chromatin and reproductive chro- 

 matin, the former being seen as large flattened peripheral masses and 

 the latter, as smaller spheroidal granules. 



In all except protociliates and a comparatively small number of 



