34 THE PROTOZOA 



As regards material, the shells may be composed of elements 

 secreted by the organism (" autophya," Haeckel), as in Hyalosphenia 

 (Fig. 16, B), or of foreign particles taken up by the animal from its 

 surroundings ("xenophya "), as in Difflugia (Fig. 16, A). Skeletal 

 elements secreted by the organism may be of organic or inorganic 

 nature. In the former case they are probably chitinous in most 

 cases, or composed of a substance allied to chitin ; in the latter they 

 are either calcareous or siliceous. A good example of the formation 

 of a shell is seen in Euglypha (Fig. 59), where the chitinous plates 

 composing it are formed first of all in the interior of the proto- 

 plasmic body, and pass to the surface to build up the shell. When 

 the shell is built up of foreign particles, the material employed may 

 vary greatly, and consists generally of particles of sand, grit, etc., 



B 



FIG. 16. Examples of shells or houses formed by Protozoa. A, Difflugia spiralis, 

 which forms a house built up of foreign bodies ; B, Hyalosphenia cuneata, in 

 which the house is built up of plates secreted by the animal itself (compare 

 also Euglypha, Fig. 59). Both these species belong to the order Amcebsea ; 

 the pseudopodia (ps.) are seen streaming out of the mouth of the shell. After 

 Leidy ; A magnified 250, B 500 diameters. 



taken up at hazard from the environment. Such shells are de- 

 scribed technically as " arenaceous." In the case of Difflugia, 

 Verworn (36) was able to cause it to build up its test of various 

 materials, such as particles of coloured glass or other substances, 

 when these were supplied to it exclusively. Many species of 

 Foraminifera, however, form their tests exclusively of particular 

 materials under natural conditions. Thus, in the genus Haliphy- 

 sema (Fig. 17) the test is formed of sponge-spicules ; in Technitella 

 thompsoni the calcareous plates of echinoderms are selected ; and 

 other instances could be cited in which the organism selects habitually 

 for its shell certain materials from a varied environment in which 

 the particular materials required may be far from common in 

 occurrence relatively to other particles apparently equally suitable 

 (see especially Heron- Allen and Earland). Verworn (36) found that 



