STRUCTURAL DIFFERENTIATIONS 129 



and ciliates, the cortex is the seat of precipitation of different 

 mineral substances; of secretion of gelatinous substances; or of 

 protoplasmic modifications into lifeless organic substances of various 

 kinds. These ^'arious products of cortical activity are moidded 

 into close-fitting, lifeless membranes of chitin, pseudochitin, and 

 cellulose, or into loosely-fitting shells, tests, skeletons, cups, tubes 

 and the like. These are not di^'ided when the cell divides but are 

 either left as empty shells and tests, or one of the daughter indi- 

 viduals after reproduction remains in the old shell while the other 

 individual makes a new shell for itself. 



Gelatinous mantles are common in flagellates and are occasionally 

 found in the ciliates {e. (/., Ophrydium versatile), but gelatinous 

 materials are secreted by all types of Protozoa. Usually, when the 

 secretion is abundant, daughter cells remain embedded in it as a 

 matrix after division, and the so-called spheroidal types of colony 

 result (see p. 34). 



The most characteristic shell-forming material manufactured by 

 Protozoa is chitin and pseudochitin. Chemically chitin is a modified 

 protein (C30H50O10N4 or multiple) and undoubtedly polymorphi 

 in composition. Its mode of formation is still uncertain but condi- 

 tions in Protozoa support the view of Chatin that it arises by trans- 

 formation or differentiation of the peripheral cellular protoplasm. 

 Not only are cups, tests, "houses" of various kinds formed of 

 these substances, but cyst membranes, spore capsules of the Sporo- 

 zoa and "central capsules" of the Radiolaria as well, while impreg- 

 nated with calcium carbonate, silica, strontium sulphate, etc., 

 or covered by foreign bodies of different kinds, the chitinoid mem- 

 branes furnish the framework for the up-building of the most 

 complex shells and skeletons. In encysting ciliates the animal 

 becomes spherical, and much condensed and is surrounded by 

 an envelope of fluid-like material which condenses more and more 

 with exposure until the definite membrane, impervious to moisture 

 and resistant to all unfavorable conditions of the environment, 

 results. In Hadiolaria the central capsule is a spherical wall of 

 chitin, separating the endo])lasm from the external ])r()topIasm and 

 perforated in various ways to permit of communication between the 

 different regions of the cell (see p. 343). 



In flagellates and ciliates the chitinous houses, tests, cups, etc., 

 are usually colorless and very transparent, but in the rhizopods this 

 is unusual, the chitin shells being colored by oxides of iron usually 

 red or brown {Arcella sp. etc.). In the majority of fresh water 

 rhizopods the outer surface of the chitinoid shell is covered by foreign 

 particles of various kinds, such as sand crystals, diatom shells, or 

 even living alga^ which are glued to the membranes by a chitinous 

 cement. Similar shells, which are generally known as arenaceous 

 shells, are found amongst the Foraminifera. In other cases, plates 

 9 



