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 epithelium (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 to7ne or sac-like form in which the 

 layer of cells surrounds a common space (or coele^ 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. I. — Diagram of 

 A Monodermic Organism. 



Fig. 2. — Diagram of 

 A DiDERMic Organism. 



Ectoderm., 

 •Archicoele. 



Endoderm., 

 .Archiderm. 



Archenteron.i 



Mouth. 



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 sur- 

 round a cavity called the archicoele. In the second type 

 the body is formed of two epithelia, when it is known as 



^ a low colonial protozoan belonging to the Mastigophora. 



