24 STRUCTURAL UNITS. 
The members of the lowest group or phylum, called 
Protozoa, consist of single cells, or colonies of single cells, 
whereas all the higher animals are multicellular or consist 
of cell-aggregates, The study of cell-structure is Histology 
(see Chapter IV.). 
The second structural unit is the epzthelium (derm) or 
layer of cells. A number of cells are aggregated together 
and all perform the same common function. No animal 
organism is entirely of this form, but many organs show 
this stage very clearly. 
The third unit is the éome or sac-like form in which the 
layer of cells surrounds a common space (or ce/e) and forms 
a complete organ separated from the parent-layer. 
These three stages can be traced more or less clearly 
in most organs and organisms. Their mutual relationship 
may be made more clear by a comparison with a brick, a 
wall, and a room, respectively. 
Amongst multicellular animals we can distinguish three 
important types according to their construction. The 
simplest are those with a single layer or epithelium of 
cells, called monodermic (or monoblastic). 
Fig. 1.—DIAGRAM OF Fig. 2,—DIAGRAM OF 
A MONODERMIC ORGANISM. A DIDERMIC ORGANISM. 
This is a very simple condition found in only a few 
types, such as the blastula larva and Volvox.* The single 
epithelium of cells is called the archiderm, and may sut- 
round a cavity called the archicwle. In the second type 
the body is formed of two epithelia, when it is known as 
* A low colonial protozoan belonging to the Mastigophora. 
