AN INTRODUCTION TO ZOOLOGY. 



The differences between the Protozoa and the rest 

 of the animal kingdom are much greater than those 

 existing between any other two of the nine divisions^ 

 In some classifications, therefore, this fact is expressed 

 by including all the others under the term Metazoa, 

 The former are distinguished by their comparative 

 simplicity, while the latter have specialized reprodac 

 tive cells, and a body composed of those definite layers 

 of cells which have been already named (pp. 30, 112). 

 Tiiese are the ectoderm and endoderm, with the addi- 

 tion, in all but the lowest forms, of the mesoderm. 

 Some authors prefer to distinguish several grades in 

 the Metazoa, the Coelenterata forming the first grade, 

 the lowest worms the second, and the rest of the 

 Metazoa the third. The first of these grades, i.e. the 

 Coelenterata, has no separate body-cavity, and is, in 

 its lowest forms, practically composed of only the two 

 primary layers. In the next, that of the lowest worms, 

 n, separate body-cavity is also lacking, but the inter- 

 mediate layer is present. The remaining grade com- 

 prises that of animals which have a tliree-layered body, 

 and also a body-cavity of complicated formation. 



Different names have been given, by different 

 framers of classifications, to the various large groups, 



coveries may necessitate their modification. For instance, a, 

 theory has recently been suggested tliat the ancestors of 

 Vertebrates ■were much more nearly related to Crustacea than 

 has been hitherto supposed; it Tvill be obvious that the 

 acceptance of any such theory would necessitate a consider- 

 able alteration in the Tree. 



