CHAPTER IV 



SUBKINGDOM II. METAZOA 

 TYPE II. CCELENTERATA 



All animals except the Protozoa may be placed under the 

 subkingdom Metazoa (Gr. fierd, after, and £<oov, animal), the 

 higher animals, as they are usually called. They are all multi- 

 cellular organisms, and the cells are always differentiated so as 

 to form at least two layers, an outer and an inner, known 



Fig. 33. Diagrams illustrating the different types of cell arrangement in the Metazoa. 

 A, outer layer of cells, ectoderm ; inner layer of cells, entoderm ; between the two, basement 

 membrane. B, outer layer of cells, ectoderm ; inner layer of cells, entoderm; between the 

 two, mesenchyme ; basement membrane and mesenchyme are often called together, meso- 

 glaea. C, outer layer of cells, ectoderm ; innermost layer of cells, entoderm ; layer in con- 

 tact with the ectoderm, somatic mesoderm ; layer in contact with the entoderm, splanchnic 

 mesoderm ; between the two, the body-cavity or ccelom. (Drawn by the author.) 



respectively as the ectoderm and the entoderm, which differ in 

 function as well as in position. These two layers are never in 

 direct contact, but are separated by a third structure, which 

 exhibits several types of differentiation. In its simplest condi- 

 tion it is merely a gelatinous or fibrous secretion, which may be 

 called a basement membrane. In other cases there is a gela- 

 tinous secretion containing cells, usually stellate or spindle- 



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