38 



PROTOZOA. 



pseudopodia is almost always absent, and the body is generally 

 bounded by a thin pellicle or cuticle. Certain fixed Ciliata, as 

 Stentor (Fig. 31) and Cothurnia, secrete external coverings or shells 

 into which they can be retracted. Nourishment is in a few cases 

 taken in by endosmosis through the whole surface of the body ; 

 but as a general rule there is an oral aperture usually near the 

 anterior pole of the body, through which solid food is introduced. 

 A second aperture, which acts as anus, and which can be seen as 

 a slit durfhg the exit of the excreta, is often 

 present at a definite part of the body. 



The pellicle seems to be part of the living 

 tissue, and frequently has an alveolated struc- 

 ture, in which case it forms the alveolar 

 layer. Beneath it there is generally a layer 

 of cortical plasma the ectoplasm which has 

 a firmer consistence than the more fluid endo- 

 plasm. The pharynx, or oesophagus, projects 

 into the endoplasm as a tubular prolongation of 

 the ectoplasm and pellicle. Through this the 

 food-stuff passes into the endoplasm, in which 

 it gives rise to food-balls. The latter undergo 

 a slow rotating movement round the body in 

 the endoplasm, during which the food is 

 digested; and finally the solid useless remain- 

 der is ejected through the anal aperture. A 

 digestive canal, bounded by distinct walls, exists 

 .. no more than do the numerous stomachs which 



riG. ol. stentor rcesehi 



Ehrb., after stein, o Ehrenberg, who was deceived by the food vacu- 



oral -P-ture^ith guU ^ ^^ tQ ^ ^^ po l ygastri ^ In 



vacuoie ; N nucleus. some cases there is a tube leading in from the 

 anus towards the oesophagus (Nyctotherus), but 

 it ends in the digestive endoplasm, and does not join the oasophagus. 

 In some cases (the EncJielina and Chlamydodonta, Fig. 32), the 

 oesophagus is surrounded by a layer of stiff rods forming the so- 

 called rod-apparatus of the oesophagus. 



The firmer, more viscid ectoplasm is to be regarded pre-eminently 

 as the motor and sensory layer of the body. In it we may find 

 fibrillse resembling muscular fibres (Stentor, stalk of Vorticella). These 

 fibrillaB are differentiations of the alveolar layer and are sometimes 

 varicose. When they shift into the cortical layer (ectoplasm), they 

 form the so-called myophane layer. Small rod-shaped bodies the 



