THE LOBOSA 85 



these punctuations appear to be globular in shape, but their precise 

 nature has not yet been determined. 



In Quadrula the outer sheath consists of a series of square 

 plates cemented together by the matrix. These plates can be 

 raised to a high temperature without destruction of their form. 

 When boiled for a long time in 10 to 20 per cent KHO, they 

 are dissolved but leave behind a fine granular residue which 

 probably represents the inorganic components of the plates. In 

 Nebela the plates are discoidal, and in other genera irregular in form. 



The diatom or desmid shells, the grains of sand or glass, and 

 other foreign bodies that are found fastened to the outer sheath of 

 the test of Difflugia (Fig. 20) and its allies are not adventitiously 

 placed, but are caught and definitely arranged in position by the 

 animal (Rhumbler). There can be little doubt that Difflugia exercises 

 a deliberate choice of the particles it uses for shell purposes, and 

 to a certain extent the character of the foreign particles and their 

 arrangement can be used for racial or specific distinctions. 



In the Arcellidae the outer sheath is composed of hexagonal or 

 irregular prisms (Fig. 1 8), some of which, situated at regular 

 intervals, are rather longer than the others and 

 project on the surface as round knobs or bosses. QcOnxcnnnmj 

 The prisms are cemented together by an extremely FIG. is. 



thin matrix. Section through the 



The cytoplasm of the Thecamoebida is often BSSSttEB 

 arranged in three zones. The cytoplasm of the som . e of ^ hich project 



, , . -i i- -i r i i . -it irregular intervals 



pseudopodia and ot the region of the pylome is as shallow bosses on 

 usually remarkably hyaline and the granulations Awerin U zew?)' 

 extremely fine. In the middle zone it is more 

 coarsely granular, and contains the contractile vacuoles, food- 

 vacuoles, crystalline bodies, excreta, oil-globules, etc. In the zone 

 next to the fundus of the shell is usually found the nucleus or 

 nuclei and the sickle-shaped or more irregularly disposed chromidial 

 network. In the Arcellidae, however, the arrangement is somewhat 

 different from this (p. 86). 



The pseudopodia are probably subject to considerable variation 

 in shape and number according to external conditions. In the 

 Difflugiidae there may be only one long finger-like pseudopodium 

 extended to a length double that of the shell, or there may be 

 three or four shorter pseudopodia, or occasionally as many as seven 

 protruded at the same time. In Heleopera the number of pseudo- 

 podia appears to be constantly more numerous than in other genera 

 of the family. 



In some species of Arcellidae and Cochliopodiidae a membranous 

 expansion of the cytoplasm sometimes protrudes from the pylome. 

 Very little is known concerning the contractile vacuoles of the 

 Thecamoebida, as the thick opaque test interferes considerably with 



