40 PROTOZOOLOGY 



in 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 substance may in many forms be 

 produced to surround the shelled body or in the members of Volvo- 

 cidae to form the matrix of the entire colony in which the individuals 

 are embedded. In the dinoflagellates, the shell is highly developed 

 and often composed of numerous plates which are variously sculp- 

 tured. 



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 ways which characterize the different gen- 

 era. Newly formed shell is colorless, but older ones become brownish, 

 because of the presence of iron oxide. Difflugia and related genera 

 form shells by gluing together small sand-grains, diatom-shells, 

 debris, etc., with chitinous or pseudochitinous substances which 

 they secrete. Many foraminiferans seem to possess a remarkable 

 selective power in the use of foreign materials, for the construction of 

 their shells. According to Cushman, Psammosphaera fusca uses sand- 

 grains of uniform color but of different sizes, while P. parva uses 

 grains of more or less uniform size but adds, as a rule, a single large 

 acerose sponge spicule which is built into the test and which extends 

 out both ways considerably. Cushman thinks that this is not acci- 

 dental, since the specimens without the spicules are few and those 

 with a short or broken spicules are not found. P. howmanni, on the 

 other hand, uses only mica flakes which are found in a comparatively 

 small amount, and P. rustica uses acerose sponge spicules for the 

 framework of the shell, skilfully fitting smaller broken pieces into 

 polygonal areas. Other foraminiferans combine chitinous secretion 

 with calcium carbonate and produce beautifully complicated shells 

 (Fig. 4) with one or numerous pores. In the Coccolithidae, variously 

 shaped platelets of calcium carbonate ornament the shell. 



The silica is present in the shells of various Protozoa. In Euglypha 

 and related testaceans, siliceous scales or platelets are produced in 

 the endoplasm and compose a new shell at the time of fission or of 

 encystment together with the chitinous secretion. In many helio- 

 zoans, siliceous substance forms spicules, platelets, or combination 

 of both which are embedded in the mucilaginous envelope that 

 surrounds the body and, in some cases, a special clathrate shell com- 

 posed of silica, is to be found. In some Radiolaria, isolated siliceous 

 spicules occur as in Heliozoa, while in others the lateral development 

 of the spines results in production of highly complex and the most 



