238 
COMPARATIVE ZOOLOGY. 
the family Bovidce , genus Bos , species Taurus ; the Deer 
is of the family Cervidw , genus Cervus , species Yirgini- 
anus , if the common Deer is meant. 
The diagram on the opposite page roughly represents 
(for the relations of animals cannot be expressed on a 
plane surface) the relative positions of the subkingdoms 
and classes according to affinity and rank.* 
SERIES I.—PROTOZOA. 
Animals without cellular tissues (the body consisting of 
a single cell), and with no true eggs. The body which 
corresponds to the egg does not develop a blastoderm. 
Subkingdom I. —Protozoa. 
This division was proposed by Yon 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. Though the division was at first 
artificial and provisional, the name now has a very definite 
signification. The classes composing it are not founded 
on a common type, but are distinguished by the absence 
rather than the presence of positive characters. Many 
stand parallel to the Protophyta 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 physical 
forces. They have no cellular organs or tissues, yet they 
take and assimilate food, grow, and multiply, which are 
* The student should master the distinctions between the great groups, or 
classes, before proceeding to a minuter classification. “ The essential mat¬ 
ter, in the first place,” says Huxley, “ is to be quite clear about the different 
classes, and to have a distinct knowledge of all the sharply definable modifi¬ 
cations of animal structure which are discernible in the Animal Kingdom.” 
