38 PROTOZOOLOGY 



cytoplasm which is more fluid. However, the majority of Proto- 

 zoa possess a characteristic and constant body form due to the 

 development of a special envelope, the pellicle. In Amoeba striata 

 and A. verrucosa, there is an extremely thin pellicle. The same is 

 true with some flagellates, such as certain species of Euglena, 

 Peranema, and Astasia, in which it is elastic and expansible so 

 that the organisms possess a great deal of plasticity. 



The pellicle of a ciliate is much thicker and more definite, and 

 often variously ridged or sculptured. In many, linear furrows 

 and ridges run longitudinally, obliquely, or spirally; and, in 

 others, the ridges are combined with hexagonal or rectangular 

 depressed areas. Still in others, such as Coleps, elevated platelets 

 are arranged parallel to the longitudinal axis of the body as four 

 girdles. In certain peritrichous ciliates, such as Vorticella moni- 

 lata, Carchesium granulatum, etc., the pellicle may possess nodu- 

 lar thickenings arranged in more or less parallel rows at right 

 angles to the body axis. 



While the pellicle always covers the protozoan body closely, 

 there are other kinds of protective envelopes produced by Proto- 

 zoa which may cover the body rather loosely. These are the shell, 

 test, lorica or envelope. The shell of various Phytomastigina is 

 mainly made up of cellulose, a carbohydrate, which is widely dis- 

 tributed among the plant kingdom. It may be composed of a 

 single or several layers, and may possess ridges or markings of 

 various patterns on it. In addition to the shell, gelatinous sub- 

 stance may in many forms be produced to surround the shelled 

 body or in the members of Volvocidae to form the matrix of the 

 entire colony in which the individuals are imbedded. In the dino- 

 flagellates, the shell is highly developed, and often composed of 

 numerous plates which are variously sculptured. 



In other Protozoa, the shell is made up of chitin or pseudo- 

 chitin (tectin). Common examples are found in the testaceans; 

 for example, in Arcella and allied forms, the shell is made up of 

 chitinous material, constructed in particular w^ays which char- 

 acterize the different genera. Newly formed shell is colorless, but 

 older ones become brownish, because of the presence of iron oxide. 

 Difflugia and related genera form shells by glueing together small 

 sand-grains, diatom-shells, debris, etc., with chitinous or pseudo- 

 chitinous substances which they secrete. Many foraminiferans 

 seem to possess a remarkable selective power in the use of foreign 



