42 INVERTEBRATE ANIMALS. 
holes through which the pseudopodia are emitted, and it is 
usually of extreme beauty, being sculptured in various ways, and 
often adorned with spines. The sarcode of the body is usually 
olive brown in colour, and often does not quite fill the shell. 
The pseudopodia are filamentous, and exhibit a slow circula- 
tion of granules along their borders, but they do not run into 
one another. All the Polycystina are microscopic, and they 
are all inhabitants of the sea. They are best known to students of 
the microscope as the “ Fossil Infusoria of Barbadoes,” as they 
occur in incalculable numbers in a sandstone in that island. 
In the third family (Thalassicollida, fig. 9) are included a 
number of singular gelatinous organisms which may be as large 
as an ordinary marble, but are often hardly visible to the un- 
assisted eye. They are found floating passively at the surface 
of most seas, and a few forms have been described from fresh 
waters. 
Fig. 9.—a Siliceous shell of Collosphera ; 6 Thalassicolla, showing the radiating 
pseudopodia and groups of siliceous spicula (after Miiller). 
The body in all the Zhalassicollida is composed of sarcode, 
and has the power of giving off threadlike radiating pseudo- 
podia, which sometimes run into one another and form networks, 
Generally the sarcode also exhibits spherical, bladder-like bodies 
or vesicles, the exact nature of which is obscure. In all cases 
the sarcode-body appears to have the power of secreting flint 
in some form or other. In Collosphera (fig. 7, a), the flint is 
secreted in the form of a shell or test, perforated by large aper- 
tures. In Zhalassicolla (fig. 7, 5), the silica forms groups of 
needles or “‘spicula,” scattered here and there in the jelly-like 
sarcode. 
ORDER V. SponcipA.—The last order of the Rhizopoda is 
that of the Sjongida, the exact nature and position of which 
have only recently been determined. For a long time sponges 
