PROTOZOA. 231 
Subkingdom.—Prorozoa. 
This division was proposed by Von Siebold in 1845 to 
contain that vast cloud of microscopic beings on the verge 
of the Animal Kingdom which could not be received into 
the other subkingdoms. It is artificial and provisional. 
The classes composing it are not founded on a common 
type, but are distinguished by the absence rather than the 
presence of positive characters. The time may come when 
the microscope will resolve these nebule, so that we can 
give them a natural classification. Probably some of them 
are transitory stages in the history of higher organisms. 
Many stand parallel to the Protophytes of the Vegetable 
World, and no definite line can be drawn between them. 
Protozoans agree in being minute, aquatic, and exceed- 
ingly simple in structure, their bodies consisting mainly 
or wholly of the contractile, gelatinous matter called pro- 
toplasm, or sarcode —the first homogeneous substance 
which has the power of controlling chemical and phys- 
ical forces. No traces of nervous or muscular fibres, cir- 
culatory or digestive organs have been discovered. Yet 
they take and assimilate food, grow and multiply, which 
are the essential signs of life. The usual methods of re- 
production are self-division and budding. 
The subkingdom may be divided into four classes: Gre- 
garinida, Rhizopoda, Infusoria, and Spongida. The per- 
fectly homogeneous Monera of Haeckel would rank lower 
than the Gregarinee; but as they are doubtfully referred 
to the Animal Kingdom, we do not include them. 
Cuass I.—Gregarinida. 
The Gregarinz, discovered by Dufour in 1828, are the 
simplest animal forms of which we have any knowledge. 
They closely resemble a cell, or microscopic egg; the only 
organ is a nucleus, suspended in extremely mobile granu- 
