18 BRITISH FRESHWATER RHIZOPODA. 
suggested that in the case of Huglypha 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 dentistoma, as appears by 
its breaking, under pressure, with an irregular frac- 
ture. Simple chitinous membrane, as seen in most 
Nebelx, etc., does not fracture when subjected to pres- 
sure. The homogeneous plane tests of Arcella, 
Cyphoderia, and some others, are finely punctated. 
Pseudochlamys secretes a simple protective shield, 
which, in the earlier stage of the animal’s existence, is 
Fig. 9.—Transparent test of Sphenoderia fissirostris, composed of 
large oval plates. x 375. Fie. 10—Transparent test of 
Quadrula symmetrica, composed of quadrangular 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 Arcelle, 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 Diffiigix, 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 
