18 BRITISH FRESHWATER RHIZOPODA. 



suggested that in the case of Euglypha the secretions 

 take place in the ectoplasm. 



In some Rhizopoda (e. g. Sphenoderia) the test is 

 apparently siliceous. Silica also enters into the com- 

 position of the test of Nebela deutistoma, as appears by 

 its breaking, under pressure, with an irregular frac- 

 ture. Simple chitinous membrane, as seen in most 

 Nebelgs, etc., does not fracture when subjected to pres- 

 sure. The homogeneous plane tests of ArceHa, 

 Cyphoderia, and some others, are finely punctated. 

 Pseudochlamys secretes a simple protective shield, 

 which, in the earlier stage of the animal's existence, is 



11 



FIG. 9. Transparent test of Sphenoderia fissirostris, composed of 

 large oval plates. x 375. FIG. 10. Transparent test of 

 Quadrula symmetrica, composed of quadrangiilar plates, x 375. 

 FIG. 11. Ordinary form of test of Trinema enchelis. x 375. 



flexible and susceptible of constant change in sympathy 

 with the creature's movements. It is not improbable 

 that some young Arcellse, covered with the same kind 

 of filmy envelope or rudimentary test, are capable of 

 undergoing similar mutations. Forms are sometimes 

 met with which it is difficult to interpret on any other 

 hypothesis. 



The Difflugise, as previously stated, have, in most 

 species, sand-grains and other substances selected from 

 the surrounding medium, superimposed upon a secreted 

 membranous base. Experiments have been made from 

 time to time, by Continental naturalists, upon these 

 structures, with results which are both interesting and 

 suggestive. Verworn (1888) directed, his attention 



