STRUCTURAL DIFFERENTIATIONS 127 



specialized adaptations and the source of many important organoids 

 of the cell. Here it is quite different from the inner protoplasm in 

 structure and in function and the aggregate, to distinguish it from 

 the relatively simple ectoplasm of Amoeba is better known as the 

 cortical plasm, or simply the cortex. 



1. DIFFERENTIATIONS OF THE CORTEX. 



It is quite probable that there is no such thing as an entirely 

 naked cell among the Protozoa. Even in Amoeba jjroteiis, the 

 classical example of a naked cell, the ectoplasm is covered by a 

 delicate, viscous hyaline zone of modified protoplasm. Hofer, 

 Verworn, and others, ha\-e noted it in connection with food taking; 

 Schaeffer (1917), in connection with movement claiming that it 

 is a third kind of protoplasm in addition to ectoplasm and endo- 

 plasm and Chambers (1915) came across it in connection with micro- 

 dissection experiments. Among Sporozoa and Infusoria it has been 

 described in many species, and in flagellates and ciliates it is not 

 infrequently characterized by definite markings or sculpturing. It 

 is the most external portion of the cell and is distinguished from 

 the remainder of the cortex by the special name periplast or pellide. 



The periplast always fits the bod\' closely, dividing when the 

 bod\' divides, thus differing from all other types of lifeless coverings; 

 in Paramecium caudatum, for example, during plasmolysis, it 

 becomes separated from the rest of the cortex and distended by the 

 accumulation of fluids. In other cases it is much more definite and 

 membrane-like as in C'ocA//o/;or//^/m bilimbosum (Fig. 8, p. 30), or in 

 the loricate ciliates such as Euphtes harpa, Vronychia setigera and 

 their allies. Periplasts are frequently delicate enough to give way to 

 forces generated within the body, but elastic enough not to break, 

 a phenomenon resulting in peristaltic movement which is not infre- 

 quent in Gregarinida (e. g., Monocystis agilis) and in flagellates 

 (Euglenida). Such organisms are said to be "metabolic" and the 

 peculiar motion is sometimes called "euglenoid movement." 



In many cases the periplast is ornamented by striations which 

 usually run obliquely down the cell {Phacus longicaudus, Eiiglena 

 oxyuris, Fig. 65); in some cases by ridges {Phacus pyriim, Chloro- 

 peltis sp. Menoidium incurimm, etc.. Fig. 65, D, F); by furrows or by 

 nodules as in the ciliate Vorticella monilata. In Coleps hirtus the 

 periplast is differentiated into definite plates of characteristic form 

 arranged in four girdles which compose an armature for the organism 

 (Fig. 65, A, C). The skeletal structures of Diplodiuium ecaudatus 

 are likewise differentiations of the periplast (p. 20). 



Not only the periplast, but the entire cortex has become dift'eren- 

 tiated in a great variety of ways in response, apparently, to the 

 many demands made upon it as a result of its contact with the 



