n6 AN INTRODUCTION TO ZOOLOGY 



The important thing to bear in mind is that too much weight 

 must not be given to the classification of any particular animal, as 

 it merely represents the present state of our knowledge, and if fresh 

 facts come to light might have to be revised. The animal itself is 

 the thing with a real existence, and the various groups, species, 

 genera, etc., are merely convenient abstractions that enable the 

 zoologist to deal with and arrange an enormous mass of material. 



Let us now consider the way in which the animal kingdom 

 is classified. It has been noted above that the most primitive 

 animals consist of one cell, and these simple forms are grouped 

 together in the Phylum Protozoa, the unicellular animals, and 

 separated from all the remaining forms, which are termed the 

 Metazoa or multicellular animals. The Protozoa are not, strictly 

 speaking, unicellular, for although the majority of them consist of 

 only one cell, a large number are composed of many cells joined 

 together. In this case, however, the cells are all more or less similar 

 and co-equal, and they stand in marked contrast to the cells of the 

 Metazoa, which are always differentiated to form tissues. The two 

 groups may be distinguished as cell animals and tissue animals. 



The lowest members of the Metazoa are animals composed of two 

 layers of cells : an outer layer, the ectoderm, forming the outside 

 covering of the animal ; and an inner layer, the entoderm, which 

 forms the lining of the one internal cavity of the gut or enteron. 

 They are classified together in the Phylum Ccelenterata. The remain- 

 ing Metazoa have not only the two layers, ectoderm and entoderm, 

 but a third layer of cells, the mesoderm, is also present between them. 

 The most typical forms have not only an enteron, but also another 

 cavity situated in, and surrounded by, the mesoderm, termed the 

 coelom, and hence they are distinguished as the Coelomata. 



This division, the Coelomata, is in itself subdivided into two groups : 

 the Invertebrata and Vertebrata, the former composed of a number 

 of separate Phyla. The term Vertebrata, of course, actually implies 

 that the animal 'possesses a dorsal column of separate vertebra, 

 like the frog, and this was how the term was originally employed. 

 It was subsequently found that this segmented vertebral column is 

 always preceded in development by a long unsegmented supporting 

 rod of peculiar structure, called the Notochord or Chorda dorsalis. 

 Moreover, some of the lower members of the group, i.e. Amphioxus, 

 and some of the primitive fishes retain a notochord throughout their 

 entire life, and never reach the stage of development in which it is 

 replaced by segmented vertebrae. The important distinction 

 between the two groups is then the presence of a notochord, and so, 

 strictly speaking, the term Chordata should be given to the group 

 and Vertebrata reserved for those members of it with the spinal 



