TEACKS AND MAEKINGS — PEOTOZOA. 
15 
The Protozoa are animals of simple structure and usually 
of minute size. In tliem the soft substance of the body is 
not divided into cells, such as build up the body-tissues of 
all other animals, and consequently they have no definite 
tissues. They have been called unicellular animals, but a 
single Protozoon often comprises more than is to be found 
in any one cell of a multicellular animal. In the latter 
each cell is trained for some special service and has dropped 
the qualities not essential to that service, whereas the body 
of the Protozoon has to fulfil all functions of the animal 
economy. Microscopic investigation, moreover, has shown 
that the minute drop of viscous protoplasm which con- 
stitutes the body of even the simplest Protozoon has really a 
most complicated structure. By keeping this in mind we 
shall better appreciate the significance of those exquisite 
skeletons formed by the Foraminifera and lladiolaria and 
frequently preserved as fossils. 
A fuller account of living Protozoa is given in the Guide 
to the Coral Gallery in the Department of Zoology. It is 
there explained how in some Protozoa the outer surface of 
the body is hardened, so that the animal retains always a 
definite shape, and can usually take in food only at one 
permanent opening. Further, the surface is generally pro- 
vided with definite lash-like or hair-like processes capable 
of rapid rhythmic movement. Those Protozoa are called 
Corticata, and since they have no skeleton to be fossilised, 
need no further mention here. The rest of the Protozoa 
have no hard skin, so that either the whole body may cliange 
its form within certain limits, or portions of its protoplasm 
may be extended as lobes or threads and again withdrawn. 
By stretching out a lobe, and then as it were flowing into it, 
the animal moves, and therefore these extensions are called 
pseudopodia (false footlets). Fragments of food are caught 
up in them and taken into the central body at any point. 
These Protozoa are called Gymnomyxa (naked slimes) or 
Rhizopoda (root-feet). They may be divided into the 
Classes : I. Lobosa, with lobose pseudopodia, e.g. Amoeba. 
II. Heliozoa or Sun Animalcule.s, with fine radiating pseudo- 
podia. 111. Foraminifera, with pseudopodia branching and 
again uniting so as to form a network. IV. Radiolaria, 
with fine radiating pseudopodia, as in Heliozoa, but with 
the central protoplasm enclosed in a porous membrane or 
capsule. In each of these groups there are some genera 
that construct a skeleton and others that do not ; Init in the 
Gallery 
