CHAPTER II 



THE ENTEROCCELA AND THE CCELOMOCOELA l 



1. THE DISTINCTION BETWEEN THE GRADES PROTOZOA AND 

 METAZOA. Some discussion of this subject will be found in the first 

 part of the present work. Here we start with the simplest con- 

 ception of a Metazoon, namely, a multicellular organism (i.e. an 

 organism which can be actually as well as optically resolved into 

 a number of constituent " cells " or " cytes ") in which the cell-units 

 are differentiated into at least two groups, having contrasted pro- 

 perties and functions instead of being equiformal and interchange- 

 able in function as in the multicellular Protozoa. The production 

 of micro- and macro-gametes or male and female reproductive con- 

 jugating cells does not in itself serve to distinguish the Metazoa 

 from the Protozoa, as this occurs not only in multicellular, but 

 also in unicellular Protozoa (Coccidia, Haemamcebae). The group- 

 ing of at least two different kinds of cell-units to form at least 

 two distinct permanent layers or masses in the adult organism 

 is the essential character of the Metazoa, and it does not constitute 

 a very great chasm between them and some of the aggregated or 

 multicellular Protozoa. 



2. DIVISION OF THE METAZOA INTO Two BRANCHES. The 

 Metazoa 2 are divisible into two divergent branches, which possibly 

 may be really two independent stems arising separately from 

 widely different ancestral Protozoa. These are, on the one hand, the 

 Parazoa 3 or Sponges, and, on the other hand, the Enterozoa, 4 which 

 comprise the rest of the animal kingdom. The Parazoa are charac- 

 terised by being composed of aggregates of cells, of which the outer 

 layer is protective, trophic, and reproductive in function, whilst the 



1 By E. Ray Lankester, M.A., F.R.S. 



2 The term Metazoa was introduced by Haeckel in his Studien zur Gastraea 

 Theorie, Jena, 1877, p. 12 and p. 54. Protozoa is a translation of the German word 

 "Urthiere," and was first used by von Siebold in 1841. 



3 This term is due to Sollas ; see Quart. Journ. Microsc. Sci. N.S. vol. xxiv. p. 

 614 (1884). 



4 The name Enterozoa was introduced by me in 1876 (preface to the English 

 translation of Gegenbaur's Comparative Anatomy) as a substitute for Haeckel's term 

 Metazoa. It now finds convenient application as the title for one of the two branches 

 into which Metazoa are divisible. 



